From Original War Support Wiki
1. Install proper graphics software (GIMP or Photoshop) This guide is based on Photoshop, but GIMP has the same possibilities and it’s pay-free.
2. Create a „template galery”. It should make our work much easier. Unpack few maps and export its bitmaps (As in step 4.) You should remove the grass at first – it’ll help you avoid getting it into your textures. The files which are containing the textures are named as mapname_obj.bmp
3. Create a new map in editor
4. Click Files -> Export bitmaps and choose it’s destination
5. Run your graphics software and load your file: mapname_obj.bmp
6. Change its color mode into RGB – image -> mode -> rgb color
7. Save your map
8. Load your map from the „template galery” (See step 2.)
9. Using your graphics software tools move your textures from the template into your map.
10. After your work with mapname_obj.bmp is finished change its color mode into indexed color – image -> mode -> indexed color and set up its parameters in this way []
11. Save your map and close the program.
12. Run Grndsec from your game folder
13. Set its parameters just like in this guide []
14. Files -> Load_BMP and load your mapname_obj.bmp – click autopack
15. Wait for the completion message
16. Click GBI -> save and save it into GroundGBI folder in your mod.
17. Open your map in editor – Files -> import GBI and choose your GBI from GroundGBI. Then, save your map and click Files -> restart
18. You should see your textures right now.