The seatbelts on the carts are just like in an airplane so they're really easy to use:

Fastening
To fasten your seatbelt, insert the tab into the buckle:

 

Adjusting
Adjust the seatbelt by pulling on the free end to tighten or pulling the buckle away to loosen:

 

Releasing
To release the seatbelt, lift up on the flap: