här är en guide, men den är på engelska.
1) Download aceit from train-sim.com. with it is a program called TGAtooll2A.
2) install both programs onto your PC. you will need them.
3) open TGAtool2A. browse for the ACE (texture file) you want to repaint by clicking 'load file'
4) once you have loaded the file, click 'image' on the toolbar and select 'send main image to editor'. default is MSPaint.
5) start reskinning. once you have finished just save the file.
6) exit paint and go back to TGAtool2A.
7) click 'export' and 'ACE (alpha). this is so you don't gt bugs like square wheels.
8) overwrite the original ace file.
That should be fine and dandy. Like desperate dan. If you want to edit the ALPHA channel, follow THIS tutorial.
1) retexture your image like in tutorial 1.
2) after exiting paint, click 'image' and then 'send alpha channel to editor'.
3) edit the alpha chanel.
4) once you have saved and exited, export as .ace (alpha) but DON'T OVERWRITE THE OLD TEXTURE! if the file is sscotsman03 for example, call it 'sscotsman03a'.
5) open 'AceIt!'
6) select your input file this would be 'sscotsman03a'.
7) then select your output file. this would be 'sscotsman03'.
8) then click the create texture button. you should see a box saying 'texture was created...' and aload of guff. if so then it works!
NOTE: you can't edit alpha channels of a file that doesn't have an alpha channel!