Hi @Minh-nv!
There are two ways to model such a conveyor. The first, harder way, is to make the top surface of the belt parallel to the floor. The second, simpler way is to make the middle surface of the belt parallel to the floor. It is also important to properly model the envelope for the guide bearings.
Remember that the finished belt is a cut cone and its flatten is part of the washer.

See similar model in my profile 90° belt conveyor