The best, easiest and cheapest way is to get a vinyl sticker cut, most printers do it. It will cut the shape you like and then you have to apply application tape. Then remove the backing leaving it stuck to the application tape. Once you have done that you can stick it to the wall. Remove the application tape and you have a sticker. Only problem is that sometimes it's a nightmare sticking to glossy paint. I've done hundreds of them. I wouldn't pay more than £15 - £25 for a big sticker and if you can make sure you use 3M vinyl. It's the best stuff IMO.