How can I stretch my new shoes if they have glitter on them that I don't want to come off?

If the shoes are non-shedding glitter shoes (usually the manufacturer will make this obvious as part of their branding), the glitter should stay intact when the shoes are stretched. The problem is for shoes with exposed, raised and non-sealed glitter that sheds easily, even when handling the shoes. Unfortunately, if movement, handling and wearing such shoes causes glitter to fall off, stretching will probably cause some to fall off too. However, you can try to minimize this by spraying the shoes with hair spray or spray adhesive before stretching, to try to keep most of the glitter in place. Expect some glitter to shed but if the glitter is large, gather up the lost flakes and reapply these large flakes with spray sealant––if you're happy to do some finicky repair work.