Method number #1 (Quick and messy):
1. Rename your map file to something like steek.bmp
2. Copy flippermaker gmf file you want to reskin, rename it, then open it with Notepad++ and change steel.bmp to steek.bmp (it needs to stay the same length of characters or else it corrupts the gmf file.
3. Copy the flippermaker text file, rename it something else, then change flippermaker.gmf to whatever you renamed your .gmf file.
Method number #2 (Longer and clean)
1. Rename you map file to whatever the hell you want to
2. Decompile the flippermaker.gmf with a decompiler, I personally like to use
Dummy's but you can use
Serge's which is a bit more up to date.
3. Find steel.bmp in the Decompiled gmf file, rename it to whatever the hell you named it in step one.
4. Recompile the .gmf file, rename it whatever the hell you want and put it back in your Flippermaker folder (if you want you can make a new folder to put your reskins, but make sure to change dir = Flippermaker in to the text file to the name of the new folder)
3. Copy the flippermaker text file, rename it something else, then change the flippermaker.gmf to whatever you renamed your .gmf file.