As for step by step instructions, all righty.
1. First, launch ResEdit (no-brainer)
2. Go to the file menu, and go down to 'Get File/Folder info...'
3. Locate and open the save file.
4. In the 'Type' field, enter 'GAMs' (case sensitive!) (without quotes, of course.)
5. In the 'Creater' field, enter 'Civ3' (same as above, case sensitive, no quotes)
6. Go to the file menu, and go down to Save
7. Go to the file menu, and go down to Quit.
Sorry, keyboard died before I could finish up. Anyway, if you get the lil save icon that we are all hopefully familiar with ( the lil map ), then you know you were successful.