I used to settle down for a good hour with a sponge, a bowl of water, a cloth and a razor blade. I’d leave the damp sponge pressed up against the window for thirty minutes while it loosened the sticker. Then I’d try to remove it with a mixture of pulling, rubbing and just plain scraping with the razor blade. I’d try not to remove any important body parts of mine with the blade at the same time.
I’d wash it carefully to get rid of all the dried glue, then I’d soak the new sticker. I’d float it onto the windscreen off the paper backing and it would always end up in not quite the right place. I’d move it and it would bunch up or fold over or twist. It was always near where it should be, but never at that point. I always tried to get it as close to the two edges of the corner of the windscreen to leave maximum vision but it never quite worked. Being jammed into the passenger seat of the car, working in the extreme corner of the windscreen didn’t help.