Step 1: Come up with a basic design for your engine
I started the build process of my engine with a design in Solid Works. I find it much easier to work this way, and creating parts using CNC machining processes turns out a much nicer end result. The main thing I like about using the 3D process is the ability to see how the parts will fit together before fabrication, so that I can make changes before spending hours on a part. This step is really not neccesary, as anyone with decent drawing skills can sketch out the design on the back of an envelope rather quickly. When trying to fit the entire engine into the final project, the jet bike, it will certainly help a lot.
I would also suggest that to get the best answer to questions if you are attempting to build a jet engine or turbine based project, subcribing to a user group is the way to go. The years of combined experience from various users proves invaluable, and I am a regular on the Yahoo Groups DIY Gas Turbines forum.