1. Get a package of 0000 or 00000 steel wool.
2. May sure the glass is dry.
3. Start in a corner and rub with the steel wool.
4. As the steel wool gets clogged with soap scum, unroll it to expose another section.
5. Continue until you have wiped down the whole shower.
6. Rinse, then clean using your favorite glass cleaning method.
Sparkle and Shine.