This tip really helped me as a beginner to stop wasting frames on my film; something which is really required because film is damn expensive where I live.
Ever had that moment where you take a photo and realise the lens cap was still on? You probably felt you wasted the frame so went ahead and advanced to the next frame and took the shot again. That’s where you need to stop!
If you have taken a picture with the lens cap on, guess how much light actually hit the film…none! Therefore its fine to leave the Holga how it is, take the lens cap off and re-take the shot. The frame will still be blank and ready to use.