Download your mods from the PiBoSo forum or from https://gpb-mods.com

In the older beta's you simply created bikes, tracks, etc files in the main install folder. Now you have to go to your documents folder, for example: C:\Users\[your name here]\Documents\PiBoSo\GP Bikes/mods.

There you create folders according to what you want to install. In most mods they will be pre-named so you just have to paste them into the mods folder!