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.
Step 2: Prep the Bike
In this example, I added a front caliper brake to assist the coaster brake with braking, and replaced the stock inner tubes with puncture resistant inter tubes.
Once you add the motor to this bike, the weight will increase drastically and you do not want to have to carry this thing home!
You can also personalize your bike to make it a personal fit. In this case, I powdercoated the bars and seat springs to match the whitewalls.
Step 3: Get the Motor
Don't panic, think logically. It all has to go together somehow. You might have to retrofit some pieces or custom grind others. It's all part of the game and will make victory that much sweeter.
Step 4: Test Fit the Motor on the Bicycle
This is the most important step in the process. The motor will have a lot of stressed being placed on it and it absolutely cannot be compromised. These motors have been known to fall off the frame in mid-drive when improperly mounted with catastrophic consequences. Make sure you secure it down.
Step 5: Attach the Rear Sprocket.
A near fanatical measurement process is not necessary, but could be helpful!
Step 6: Hook Up the Drive Chain!
This is also the stage where you will discover if the kit will actually fit your bike or not. Generally, if the chain does not hit the drop outs then you are good, but to be safe it's worth getting a beach cruiser frame or you will have to notch the metal where appropriate.
A side note; most of the chain tensioners that are supplied with the kits are not very good. There is a high risk of the tensioner getting sucked into the wheel and causing an accident. You might be able to get away without running one, (I did) but other have seen a high degree of success by lathing a skate board wheel and making a custom mount.
Step 7: Hook Everything Else Up!
It would probably be a good idea to run weather stripping on the wiring.
Time to drive it and be the boss on the block! Your not going to get far because everybody will want to talk to you and ask you how to build one. Putting one of these things together is an awesome accomplishment!