how to edit terrain using GUEdit: (as well as i can remember, i lost GUEdit a week ago)
1. Run GUEdit
2. Load your map
3. Click on "edit terrain"
4. It will open a "browse" window, find the little icon in your starcraft folder that starts starcraft normally, select it.
5. You will have to wait for a minute now.
6. Keep waiting
7. Two windows will pop-up, in one you can choose the square tiles of terrain you want to use, the other let's you place it on your map.
8. Exit out of both the windows, underneath the "edit terrain" button there is a button, "export terrain" or something like that, click that.
9. Save your map as a new map with a different name
10. Exit
NOTE: do not open your map again in staredit or all your work WILL be lost.