This guy does an awkward job of showing you how to set up two prussiks to ascend a rope. However you can take what you need to the climbing gym and have some thing to practice. Remember you have to practice this stuff a bit at least. Out on a glacier it is no longer crevasse rescue practice it's just crevasse rescue. So check out this climbing tutorial video and practice your safety techniques and learn how to self rescue from a crevasse using two Prussiks.