To make large custom vinyl stickers, come up with the idea for the sticker and make a sketch. Then scan the sketch, and upload it into a document in the size of the printing paper. To make a quality sticker, choose a resolution of at least 300 pixels per inch and the CMYK color space. Trace the sketch using a pen tool, then color it. Print the sticker on an adhesive vinyl sheet, and cover it with clear laminate.
Before printing the sticker, make sure that it fits completely on the vinyl sheet. Try printing it on scratch paper of the same size first, and adjust the image accordingly. If you need to print a sticker of a size that doesn't fit into your printer, find a professional printer that has the equipment needed to print in larger sizes.
Once you print the sticker, cut it out with scissors, and make final touches using a trimmer.
Use only an inkjet printer to make vinyl stickers. For picture editing, use an image editor such as Photoshop or Illustrator or free alternatives such as InkScape and GIMP. Vector graphics editors are easiest to use.
Online resources for sticker ideas include Pinterest, Flickr, DeviantArt and sticker stores.