I put a lot of 3D stuff here:
http://www.classicsteamengineering.com/index.php?PHPSESSID=ga934socs294p3djjcencp5b01&board=141.0
I made some 3D tutorials here, but they were done early in my learning curve, and some of the actual model engine designs may be better learning tools.
Edit: Note, the tutorials below are not really that good, and they were all created before I had a good grasp of 3D modeling. The 3D models created for engines in the link above are much better I think as far as useful information for modeling engines.
http://www.classicsteamengineering.com/index.php?PHPSESSID=ga934socs294p3djjcencp5b01&board=2.0
I use Solidworks, but it is exactly the same process, select a plane, draw a sketch, extrude or cut the sketch, go to the next plane, etc.
This stuff is all open-source, anyone can see it regardless of whether you are a member or not. (Members and non-members see exactly the same posts and photos).
My first 3D models were not very good, and I have since learned that it is generally best to start a sketch on one of the standard planes (front, top, or side) and then extrude from the center out. That way, if you add on to the ends of say a cylinder, and then later go back and lengthen the cylinder, then most of the associated objects such as cylinder flanges, etc. automatically stretch with the cylinder. I also extrude flywheels from the center out.
Jumping back and forth onto and off the various planes confused me to no end in the beginning. Here is what you have to remember.
Sketch planes are only for drawing 2D sketches on.
All sketches have to be completely closed, and represent the outline of some section cut through a part. Once you finish a sketch, you generally use the extrude boss or extrude cut feature, and when that feature is done, you generally are no longer on the sketch plane that you started on.
Think of sktech planes as datum surfaces, and you are setting up a piece of metal in the milling machine. Obviously, to cut on various planes, you have to rotate the piece of metal so that the correct plane is facing the cutter, or in the case of 3D, the correct plane is facing you so you can sketch on it.
You only return back to a particular sketch if you want to modify the shape of it, otherwise your 2D sketches are hidden from view so that you can see the 3D model. In 3D mode (not on a sketch plane) you can strech existing extrusions, mirror extrusions or cuts, etc.
Sketch plane is for sketches only, and only for drawing 2D sketches.
When you are in 3D mode (not on a sketch plane), you are manipulationg 3D solids by streching them, mirroring them, mirroring holes, and other manipulations of 3D objects.
Another very valuable trick is to toggle the view from a 3D solid to a transparent "lines only" isometric view, so that you can see surfaces that occur in the background. You can actually start a sketch on a plane in the foreground, and use the lines which show up in the background on the isometric as starting/ending points. This feature is worth its weight in gold.