Eggs are living things and they do need oxygen. I don't know what type of incubator you built but if it doesn't have forced air, a fan pulling air in and moving it around you need holes near the bottom and near the top. Cool air is brought in at the bottom and warms, rises to the top and leaves, pulling more air in the bottom. Small holes are all thats needed but the size and number depend on the size and shape of the incubator.
A good way is to put in more holes than you think you need but have plugs for them so you can block some. You need to keep the humidity up and too much ventilation makes that hard, but if the incubator isn't ventilated enough, you can drown the chicks in the shell because they can't get enough moisture out of the egg pores because the air in the incubator is too saturated. And at hatching time chicks need good ventilation or they can become too weak to get out of the shell. All that work reguires extra oxygen.
You will have to play it by sight with your home made incubator. If you can keep the temperature up and the humidity at the right level with several holes open then do it. If water is dripping inside open more holes. If you can't keep the temp and humidity right you need to close some holes. And at hatching time do what you can to provide more air.