1.) While in the TE, select the objects tab. There now is a list of available shapes to choose from.
2.) Select the plane or shape you want to edit then hit save texture
3.) You will find all the bitmaps (.bmp's) for the skin in C:/Program Files/HTC/aheditor/terrains/(your maps name here)/savedtex/ folder.
4.) To edit the skin you'll need an editting program to change the bitmap. Some include Photoshop and Paint Shop Pro. If you lack software there is a lot of great freeware out there, too.
5.) Now, change it from 256 color to 16 million
6.) Then using whatever painting program you have, edit the object to your liking, change it back to 256 color and save it into the texsrc folder [C:Program Files/HTC/aheditor/terrains/(your map's name here)/texsrc], which you will have to create.
7.) Now, make your map in the TE and if you did everything correctly, when you play your map the edited skins will be used.
8.) Make sure you copy the .res file from C:Program Files/HTC/aheditor/terrains/(your map's name here)/texsrc to the terrains folder in C:Program Files/HTC/ACESHI~1/terrains.
If I did anything wrong, just tell me and I will edit the post.
Thank you DiieWankee for the original.