1. First, make the shape on the second slide. The reason I chose this shape is that this way the belt will be able to go in easily.

2. Next, extrude the sketch so that the belt will have something to go into. Don't forget to leave a hole for the screw.

3. Make a sketch of the rectangular hole. Don't forget to measure the width and thickness of your belt and use those measurements.

3. Afterward, cut the rectangular hole. 

4. Last, of all fillet the edges so that you won't cut yourself while wearing this.