Just like what vexorian said, each AFOL has his/her own way of creating their MOCs.
As for me, this is how I do my MOCs:
1. Come up with an idea, then try to visualize it and finalize what you really want.
2. Get some reference picture/s to serve as a guide, particularly if you're trying to recreate a certain structure, vehicle, or scene.
3. Search for existing LEGO sets which might be similar to what you want to create. You could find some much-needed help and inspiration in the downloadable instructions in the LEGO online customer service site.
4. Make a plan. Identify the parts that you need and how many of each do you need. You don't need to be exact, an estimate will do.
5. If you have the parts that you need from your existing sets, then you're good to go. But if not, or you don't want to scrap your official sets, then Bricklink is your bestfriend.
6. Finalize your purchase of parts, then you just have to wait to arrive to you.
7. Once your parts have arrived, you're ready to build your MOC.
8. The best part of all, enjoy the building/MOCing!
This is how I do my MOCs, it's a rather structured and scientific way, but that's the best way for me. This might or might not work for you, so you just have to find a method where you're most comfortable working.
Well, we hope to see one of your MOCs soon.