What's important to remember when you do this project is that not all the bikes will be the same, there are subtle differences that you have to be aware of, and your mileage may vary as far as the degree of difficulty on assembling bikes. Have fun!
Step 1: Step One! Educate Yourself!
The bike is important! Do not get a cheap bike from Wal-Mart or Target! The construction of the frames are light and could split from the vibrations of the motor. At the very minimum, you should source a bike from a local bike store that is sturdy. A beach cruiser frame is the best option.
The motor you use is important. Most places on the internet will offer motor kits for your bike. These are universal kits and may not fit your specific application. Be careful before you order as most places have a no return policy.
The style of bike is important. Going for a vintage look or rat rod look may be your best bet.