Don't try to remove all the old glue - too much work. Remove large chunks and as much as comes off by scraping, but don't work too hard. Most existing glue will get employed in the re-gluing. On aluminum rims I use a heat gun to soften any lumps that won't come off.
Don't be intimidated and use common sense. You are going for adhesion, not perfection. You will make a mess on the sidewalls the first time but it won't hurt anything. I use acid brushes to get a liberal coating on rim and tire