To power the H-Bridge I will be using 2 1s lipo batteries. At full charge these batteries put out over 8V when connected in series. I do not need my motors running so quickly so I use the buck converter to bring the voltage down to 5V. No matter what you are using to power the H-Bridge you will need to run a common ground from the H-Bridge to your Raspberry Pi.

If you are using a power source that is in the range of 4V - 7V then you may not need to use a buck converter. In this case, the leads coming from the power source can be connected directly to the H-Bridge. However, you will still need to run an extra wire from ground on the H-Bridge to a ground pin on the Raspberry Pi.