Making a basic shirt in the Gimp

Natalia Zelmanov has covered this so well that for now I'm not going to write my own entry, but just link to hers :) Oh, and to this. And this too, for good measure.

Really flat prisms

• Rez a sphere, and set the dimple B=0.950.
• Change the shape to a cube.
• Set Z=0.010.

You now have an extremely flat prim with a square end to work with.

Especially good for creating plants, fur, and other irregular 3D shapes:

• Rez a flat prim.
• Shift-drag to copy; when you do this, the copy is left in place, while the original is moved away.
  
• Ctrl-z to move the original back into the same space as the copy.
  
• Select either prim and rotate about any axis.
• Do this a couple of times and you will see the illusion of a 3D object start to appear.