First off, Milkshape is not free. It costs $35. Significantly cheaper and much more suited to low-poly modelling than most comparable software. There are plenty of tutorials available on it.
I will be including a milkshape 3d file setup specifically for exporting to KSP that includes:
+ Materials for your part and node_collider, with shininess, specularity, and other settings setup to closely match the in-game appearance of other parts.
+ Simple mesh showing part orientations relative to KSP
+ Textures used in tutorial
Part One: Modeling
1. First step is to create a basic shape. With cylinders you can specify the number of sides, how many stacks, and how to close the ends of the shape. Make sure to check 'Snap to Grid' so your part is scaled to be exactly 1 meter square. Left click and drag from one grind intersection to another.
2. Now you need to Move your part precisely by entering the coordinates in the boxes so that its center of mass will be the same location as the origin (0,0,0 for X,Y,Z). Enter the coordinates then hit the MOVE button next to the coordinate entry fields. If your part went the wrong way, Undo, and change the affected direction to a negative.
3. Now you can Scale your simple shape to the final size of the part you are trying to create. Here I am making a 3m x 3m x 0.25m cylinder.
4. The Y axis indicator is in the way of something I need to select. So I can select it with the following options, then Hide it (CTRL + H).
5. With the Y axis now hidden, I can use select By Vertex and Ignore Backfaces to choose the top surface of the cylinder.
6. I'm going to make this cylinder into a ring, so I need to Extrude the top of the cylinder to add a stack to it. I'm going to enter a distance to extrude to, which will make moving it back to where it came from much easier later.
7. With the top face Extruded, I can now Scale it. I don't want it to move along the Y axis when being scaled, so I have checked 'Center of Mass'.
8. After moving the newly Extruded stack back down a precise amount to put it back at the same height where it came from, I can extrude again to make the inside of the ring.
9. Now I can just Delete the top and bottom of the original cylinder.
10. The inside and outside of the cylinder are going to make it difficult to select the next set of faces, so I'm going to select them and Hide them.
11. Now I can select the top of the ring easily. First I'm going to make a Duplicate (CTRL + D) of it. Then I'm going to Mirror it from top to bottom. Now I have a top and a bottom side of complete the ring.
12. Now I Unhide the inside and outside, select all verteces, and Weld them (CTRL+ W). However, the smoothing I end up with looks a little odd.
13. If I assign the top and bottom of the ring to a different smoothing group, the ring looks a bit better.
14. And if I regroup the ends of the ring, the inside, and the outside from eachother apart from each other then UV-mapping them will be much easier.