Your artwork must be the same size as the sticker you are ordering, plus some extra bleed (What's bleed?) which is trimmed off after printing. Your artwork must also be at least 300dpi or, if you prefer, 600dpi (What's dpi?).

All of our printing is done with a lithographic four-colour process (CMYK). In Photoshop you can either select 'CMYK Color' as your 'Color Mode' when making a new document, or you can select Image > Mode > CMYK Color.

Simply save the file as a JPEG, using the highest quality setting/lowest compression setting available. As an industry standard format, JPEG offers the best balance between file size and print quality.