All right, if you want the A-B-C's, best start with the beginning:
1. Start with making a drawing, preferably on a piece of paper. Mark the course clearly. And try to give a feeling for the look & feel of the proposed track.
2. Scan it and post it on this forum. People here will give you helpful pointers. And if somebody else is working on a similar idea you didn't waist any effort
3. Dig up a good Blender tutorial on the internet. There are many flavours of tutorials out there, find one you prefer.
4. Try to make something simple like a stick-figure or something in Blender. Be warned: The learning curve on Blender is very steep, but very rewording.
5. Using your drawing, start designing your track in blender. Use the wiki pages Auria pointed to.
6. Export it and try to run it in STK.
7. Also post the exported track on this forum. The community will help you to improve it. (Don't forget to post screen-shots to make everybody excited about the progress you made)
8. If it is good enough, post it on stkaddons.net.
As you see we have a very helpful community here, so don't be afraid to ask.