Air flow is an important factor in the cooling equation of a custom gaming PC. Without proper air circulation, the vital components inside are subject to complications from overheating. The basics of airflow can be narrowed down to the fans which produce the movement in air inside the case. The first place to start is the direction in which the fans move the air.
Blades and air direction
The blades of PC case fans are scooped. The concave side of the fan faces the direction in which it will move the air. This means that fans with scoops facing towards the inside of the case are intake fans, and the ones with scoops facing towards the outside are exhaust fans. Knowing which direction the air is moving will help to produce the right air pressure inside of the case. It is ideal to have case pressure be positive but as close to neutral as possible.
It is ideal for air to move unobstructed through the inside of the computer. The placement of fans along the chassis should promote that. Furthermore, heat rises, so fans placed on the top of the case should be exhaust fans. This is also a great place to put exhausting AIO cooler radiators. The fans on front of the case are usually used for intake. There is more room for fans on the front of the case, so maximizing this space helps promote the slightly positive pressure mentioned earlier.
Volume of air moved
On top of fan direction and placement, the cubic feet per minute (CFM) rating is important. This is a calculation that describes how many cubic feet of air the fan can move in one minute. Knowing the numbers associated with the fans being used in the case will allow for calculations to be made which will lay out exactly how many intake and exhaust fans a specific build will need. There are many resources online to calculate this.