I got my bag off etsy; they have all sorts of styles and sizes! Stickers are from anywhere from markets to department stores and stationary shops.
First off, clean your bag/surface you wish you stick the crap out of! Nothing worse than getting fluff or hair caught under your stickers and you end up having trailing strands of hair (unless that's what you want) or snapped off bits sticking out all over the place.
Collect all the stickers you want to have on your bag BEFORE you start sticking things down. I had to peel ones off to make room for the new cool 3D ones I ended up getting.
If you want some kind of symmetry for a more organised look then use a measuring tape/ruler to decide where the center of your bag is (if you have a feature sticker) or to help you divide your working space into quarters.
Home stretch! Like Purikura, the more the better, fill out every gap for that overkill kawaii look...
And you're done. Fill that up! Click here for a super cute tutorial by The Milk Club on how to style a clear bag!