1) think of what your sticker is gonna be, then make it in your photo editing software. should be easy if you already have an image of the character you want to make into a sticker.

2) don't forget to add a white outline to the image! it won't really look like a sticker if it doesn't have one.

3) after you have finished your sticker, make a post about it and attach the sticker through an image. after that, to place the sticker on others' posts, just post it in the comments of the posts and you're done!