If you are using a German keyboard, hold ALT and type in 092 to make an \ while using DOSBox.

1) Put SNESTL12.EXE into an easy to access folder, e.g. c:\snes
2) Copy ROMs that you want to patch into the same directory BEFORE you start the program
3) Open DOSBox

Type in:
4) mount c c:\snes
5) c:
6) snestl12.exe