Sure, assuming you have Classic UV mapper installed on your computer, the procedures I follow would go like this (in a nutshell):
1. Export your untextured model from Milkshape in OBJ format.
2. Load the OBJ format model into Classic UV mapper. At first it will look like a mess of black lines. Click EDIT, NEW UV MAP, PLANAR (usually I use the Z axis, but it will depend on the model). This will lay the model out in a form that you can see the different parts better.
3. Click EDIT, COLOR BY GROUP.
4. Click SELECT, ALL. Shortcut is CONTROL & 'A'.
5. Press the '/' key to shrink it (maybe a couple of times). Then move it into a corner. If you need to make it bigger, press '*'.
6. Click SELECT, by GROUP (not SELECT BY dropdown menu item), then select a group. Shortcut is CONTROL & 'G'. The selected group will be highlighted. Click on the center of it to move it where you want on the map. You can change the size and shape of it by grabbing one of the little black boxes on one of the edges. It is suggested to make the groups larger that you want to see more detail in with your finished map.
7. With the selection still highlighted, you can choose a different kind of UV map for that particular group. Do this by clicking EDIT, NEW UV MAP, (and then choose PLANAR, BOX, CYLINDRICAL, ETC.) It will depend on the shape of the group as to which UV map you will choose. Once you select the new UV map you want, it will ask the axis and orientation. You will likely have to experiment here to get the best map.
8. If you don't want to separate each group manually, instead of step 6, you can do the following: Click EDIT, TILE, by GROUP. This will evenly place all groups separately on the map. You could then proceed with step 7.
9. You lay out all of the groups the way you want, fitting everything into the image as best you can.
10. Save the model. Don't change any of the boxes that are ticked. I would use a slightly different name than the original model. It exports in OBJ format. At this point, the model and texture map look exactly the same.
11. Save the texture map. It usually will try to give you an odd width or depth. Change these to a 'normal' map setting, such as 1024 X 1024, or 512 X 512. The map will be adjusted accordingly.
12. One unfortunate thing with it is that it saves the texture map as a 256 color bitmap. You will need to load it into MS Paint or Photoshop or something and save it again as a 24 bit bitmap. I usually convert it to a different file format when I am done with it.
13. IMPORT the model that you created in Classic UV mapper into Milkshape.
14. In Milkshape, click the MATERIALS tab. Click NEW and add the UV map texture. Select all of the faces (Control & A). Click ASSIGN (still in the MATERIALS tab). You should see all of the different colored lines on your model.
15. Open up your painting program and start texturing the model (leave Milkshape open).
16. Paint a little on the texture and then go back to Milkshape and click EDIT, REFRESH TEXTURES to see what the changes look like on your model.
It will take some time and practice for you to get things looking the way you want. Try to avoid overlapping lines when laying out the UV map, as the same texture will be painted on both faces. To avoid this, select overlapping lines and move them out of a group over to the side. Selections are made by clicking the LMB and dragging the selected area you want.
One other drawback is you only get one UNDO, and this is done by pressing ESC. If you do make a mistake further back, you can always re-select the group and start over on that part.
Hope this is helpful. Good luck to you.
