How I assemble a BLDC motor.
Starting with a stator, a rotor, three bearings, a circlip, a circlip tool, some spacers, oil, three M4x8 bolts and a wrench for them, a hammer, an 8mm shoulder bolt, a dry washcloth, a motor pulley, a motor mount plate, maybe a tiny piece of griptape/sandpaper, and maybe a skateboard wheel bearing spacer:
First, I start with the stator
bolt it to a mount plate
grab a shoulder bolt or something else
I lightly hammer one bearing in with the head of the shoulder bolt
Then I put two drops of oil on the bearing
Then I lightly hammer in the second bearing and put a drop of oil on it
Get the rotor
Make sure the bearing slides all the way down the shaft without strong resistance. Sand the shaft a tiny bit if needed. then remove the bearing from the rotor
Put a cloth folded in half on the stator like so —
Make sure when the axle pokes out the back side that it’s not going to poke into your hand
Make sure you keep your fingers clear, the rotor will come down hard, fast, and sudden
Gently push the rotor on until it POPS down onto the cloth. The cloth prevents a crash-landing.
Pull the rotor up slightly while removing the cloth, and remove it from the mount plate.
Now, see this hole?
I fill it up with oil almost to the top. Do not fill it all the way up to the bearing seat. 60% full is ideal.
Slide the third bearing onto the shaft
Use a pulley or something else to gently tap the bearing in with a hammer.
Now it should appear as such:
Obtain a circlip and some spacers of various thicknesses
Use the spacers and the circlip to snug the rotor onto the stator
Put a drop of oil on the top of the bearing.
Now, do a hand test on the motor.
Enjoy 
