The next step would be to use the beveler to depress the leather next to the lines you have cut. Nighthawk's tooling tutorial shows some pictures of how this is done. You want the thick lines of the design to stand out, so always bevel the outside of the line with the deepest part of the depression closest to the thick band. Where two lines cross, you bevel across the one that goes under the one on top. Overlap the beveler strokes so that you don't see each individual stroke. Your goal is to leave a smooth trench. I overlap my beveler strokes by about half and then move on. If the trench is still a little rough, you can use the stylus with the spoon end to rub along the trench and smooth the marks out. Work in small areas, around 4 to 6 inches at a time. Of course, you have to prepare or "case" the leather before starting the tooling. This means dampening it and then letting it dry to the right moisture content before it will take good impressions. A belt is going to take a long time to do so you will get a lot of practice by the time you are at the other end.
When I started making sporrans, I told myself that the first 10 were practice. I kept the first one and gave another seven away before I sold my first one that I felt was good enough. I think I've come a long way since then, but still have a lot to learn.

Good luck and keep us informed of your progress.

Tom