There used to be a great tutorial on Armada2files.com for texturing in Milkshape. Since I can't find it, but I really do want to be a help here are a couple hints:
First, I am sure you know you have to do the actual drawing of the texture in another program. This can be as simple as getting a schematic off the internet and coloring it in MS Paint. It best to save it in a square resolution 8x8, 16x16, 32x32, 64x64, 128x128, 256x256, 512x512, or 1024x1024 and so on.
To get a texture into Milkshape, select the "Materials" tab on the right (or left depending on your gui settings). Click "New" from the top left of the bottom button cluster. This creates an empty slot for you to load a texture. Select the texture named "Material01" in the white list box above the picture of the sphere. You will see two buttons that say <none>. Click the top button and select your texture file (.jpg, .png, .bmp, or whatever) from the dialog box. You now have a texture assigned to the model file.
In Milkshape, textures are assigned to groups. Select a group either by clicking on the model in an ortho view while "Group" button is selected in under Select Options in the Models tab, or by selecting the group from the white list box under the "Groups" tab and clicking "Select" under The white list box. By default, selected groups appear read in wire frame view, and the rest appears white. Next, go back to the "Materials" tab with your group still selected and click "Assign". This will apply your texture to that group.
Unless there is some strange twist of fate and all eight planets are aligned, your texture will be stretched and mangled all over the model group. At worst, the entire group will take on the average color of the texture. To fix this, with the group still selected. Click on "Texture Coordinate Editor" in the "Window" dropdown menu. The dialog box that opens will show your texture overlaid with your mesh (or, if your model turned the average color, it may be that it is all condensed into one point in one of the corners). From here it is mostly art and experience, but to give you a fighting chance, Click select, then drag your cursor over the mesh (or single Point) to select it. You can used tools move and rotate, etc. to situate the points where you want them. You can also select a view (front, back, etc.) from the drop down menu and click "Remap." This will map your texture to whatever view of the model you have chosen. In the main Milkshape screen, you can select individual faces to work with in the Coordinate Editor, or you can select the points or nodes right there in the editor.
Let us know if you get stuck on any particular point.