We get a lot of questions about building race drones. Its not an easy task for someone who isn't familiar with the process, and it can be even more confusing when you try to narrow down just what parts to select and whether they will all be compatible.
There are three basic skill sets that will need to be mastered in your journey. Mechanical assembly, electrical assembly and programming/setup. Falling short in any of the three generally leaves you with a non functional product, so its important to be particular and check over your work.
But lets get back to the very start. First things first. What sort of flying would you like to do, and in what environments? Most people start their journey on a micro and learn the basics, and from there move up to a 3 inch or a 5 inch drone. The smaller it is the harder it will be to build generally, though you will be able to fly smaller ones in areas you cannot fly the larger ones.
Selecting parts can be challenging without understanding electrical ratings or sizes. When we specify a drone as being 5 inch, we are referencing the size of the propeller. That helps us establish the size of the frame. Its a good idea to pick a frame that has access to spare parts, the TBS source one is a great budget frame to start with. With plenty of room and spare parts available its a common frame selected by pilots of all skill levels mostly for flying acrobatic flights.
When you know what frame you want to use you can then look at the battery you intend to fly. For beginners we suggest a maximum of 4s (a four cell battery). Its important to note this as it will determine the motor specifications you use. Your motors size and KV rating will pair with your chosen battery to determine your performance. Sizes are measured in two sets of numbers, the first two are the diameter and the rest are the stator height. Generally most 5 inch quads are 22XX or 23XX in size, indicating a diameter of 22mm or 23mm. The last two numbers will determine the height, for example 2207 (7mm) or 2306 (6mm). The final number worth noting is the KV, and it is just as important as it determines the approximate "RPM per volt input." Obviously then the higher the number the faster the motor will turn (at the expense of more current drawn, shorter flight times). Most 4s motors are 2300KV to 2600KV or there about. If you want to build for endurance you will find that motors as low as 1700KV will still fly a 5 inch quad reasonably well, with a reduction in top end performance of course.
The only other thing worth noting for motors is their mounting pattern, which is usually 16mm x 19mm but occasionally is 16mm x 16mm or 19mm x 19mm. Most frames accommodate all types, but its worth checking before you make your purchase.
So you have your frame, motors and battery selected. You now need to look at your electronics. You have some more details now which will help you select the appropriately rated parts for the rest of the construction. The electronic speed controllers in particular (ESC). Most people these days are moving to 4 in 1 ESCs which do offer some advantages but also have the issue that if you damage one of the ESCs you have to replace them all. The ESCs need to be selected to handle your input voltage and current. Most ESCs are overrated for amperage but for a five inch drone its generally accepted that you should select a 30A rating or higher. The input voltage is the critical one especially if you are building higher than 4s. You will note the ESC below has 12 tabs for motor inputs and two main tabs for Battery input. Its important you get all your connections correct, especially polarity on the input tabs.
Usually I start the build process with a dry fitting of everything to make sure I have enough cable lengths and to approximate the layout to check for any potential hurdles. Once you are satisfied start at the motor ends, selecting the correctly sized screws for the arms and motors you are using to make sure they are not too long that they come up and damage the windings. A dab of semi-permanent loctite (not the red stuff) on the motor screw can help. At this stage check to make sure the circlip or retaining screw on the bottom of the motor doesn't foul up against the frame. Once your motors are in place run the three motor wires down the arm to the ESC. Lets assume you are using a 4 in 1. All three motor wires need to go to consecutive pads on the 4 in 1. Make sure they are not too tight, a little bit of slack is advised. At this stage it pays to look at the 4 in 1 to check the order of the ESCs. If you mount the 4 in 1 unusually you can either remap the motors in the software or adjust the physical signal cables to accommodate the orientation change. The important thing is once its built you test the motors one by one and they all respond according to the positions they should be in. The direction the motor spins is critical too, we can change that easily in the ESC software however. 
Make sure the mounting pattern suits your frame, most 5 inch frames will accommodate the standard 30mm x 30mm mounting size. Its also a good idea to isolate or soft mount the flight controller to stop vibrations from the frame interfering with the flight controller. Most flight controllers have this integrated these days.
We recommend getting a flight controller that is at least an f4 processor. More f7s are coming onto the market but don't really offer a significant advantage over f4 at this stage. You will need to look at your specific flight controller to see how it is wired up, there should be wiring diagrams and pinouts available. You will see a bunch of spots for connections, we will go over the basics ones that will be needed. If you want to make your hookup easy its best to order a combo kit as the ESC and FC will plug and play with each other. Once you know how everything connects you can usually make any ESC work with any FC however.
Onscreen display (OSD) is common on analogue systems now, and it requires that your cameras video line be routed through the flight controller (if its compatible). You then feed the video line out to your video transmitter. Different manufacturers mark this differently on their schematics. You may see "Vin"(video in) and "Vout"(video out) or "V" or "Cam" or "Vtx" or others. It can be a little tricky to discern in some cases as there isn't a universal standard. It is usually cleared up in the wiring diagrams however. Matek products have excellent documentation, you may want to check your products documentation before you purchase to make sure its straightforward. You will have a few power regulated outputs, at least 2 x 5v outputs and a number of UARTs. Your 5v lines will be needed for your control link receiver and other peripherals like camera, LEDs, GPS etc. Its worth checking to see if your FC has an onboard 5v regulator and if it does what its rated for if you intend to use it to supply more than the camera and receiver. Please note that not all connections need to be made, ESC telemetry, Buzzer, LED, Smartport, Camera control and others are all optional extras that arent necessary for the quad to fly.
The UART ports are input/output ports, and are used to send and receive information from the peripherals. Your Receiver is the main thing that needs to be hooked up to this. It generally will run a single line from the receiver to the appropriate UARTs Rx (receive) pad/plug. You may also find the UART ports marked as R1, T1 (UART 1 receive and UART 1 transmit). You usually have multiple UARTS so a variety of different devices can interface. The two most common ones are for the receiver and the video transmitter, which allows for remote control of the video transmitter. For FRSky protocols (usually D8 or D16) and F4 flight controllers you need to make sure you are wiring it up to the correct UART port. Often this is marked as "SBUS" but not always. On F7 flight controllers you can hook it up to any UART. For more information check the notes on your flight controller. 
The Bulk of the assembly should be done now. You will need to double check all wiring is secure and correct and everything is mounted down well. Antennas should be placed in optimal positions so that they cannot easily be damaged by propellers. It is recommended that you use a current limited powersupply or a fused input so that if anything is incorrect you dont damage components.
If everything is correct you should hear a series of audible alerts. Three in sequence, followed by two in sequence. The first set of three indicates the ESC is working and recognizing motors, the second set is the FC and receiver being recognized. If you get the first set sounding correctly but not the second its likely that your FC isnt powering up correctly.
The next thing to do now is to plug into Betaflight, update to the appropriate version and check your settings and test motor direction and order. Each motor should be checked independently to make sure its in the correct position and it is spinning the correct way. Motor positioning can be changed via CLI commands in Betaflight, but motor direction needs to be changed in BLHeli Suite, a separate application for ESC management. Following this you would then go through the process of binding and checking channel orders, assigning switches to the modes and testing the video link.
Once all these things are done its time for the first test hover!