Step 1: Frame
The first thing to decide when making an e-bike is which frame to use. When choosing a frame, keep in mind that the size, material, and rim options. If you are going to weld mounts to it, look for frame that are steel and not aluminum. Also keep in mind that usually only 20in BMX rims and road bike rims have threads on both sides to mount an additional sprocket to. I chose a BMX frame because it it's steel, has rims I can mount to, is light weight, and compact so it can fit into a car.
Step 2: Electronics
The next thing to consider is what electronics to use. Keep in mind price, size, speed and distance you want to travel on one charge. I was given my electronics for free so I did not have any options, but my bike has:
Motor- 500W 30A
Battery- 26Ah sealed lead acid
Speed Control- Alltrax 300A 24-48V
Throttle- Currie 0-5K ohm potentiometer thumb
Step 3: Mounting
There is a few ways to mount electronics, you can bolt things on or weld usually. I used 1/8in thick L-bar from Ace Hardware. I measure out the batteries and welded 2 mounts that the batteries fit snugly into. I then made a similar mount for the motor with slotted holes drilled for the motor to change chain tension. Once the mounted were made, I measured and sanded the frame where they will be fitted. The final step I did was masking the frame off so I could paint over the unfinished metal components.