1. Move your Xbox profile onto a USB drive (System Settings -> Storage)
2. Download the vehicles onto the same Content folder in a USB drive
3. Install Horizon: https://www.wemod.com/horizon (Be careful, it tries to trick you into installing an extension and some nonsense)
4. In Horizon, change the profile for each vehicle to you, then save them. I've made a video here:
5. Move your profile back to your console hard drive if you want (you should be able to move the vehicles too) and they should be loadable in-game now on your storage device of choice.

[Vehicle downloads can be found in the resources section on the sidebar]