First, I measured one little colored square on the cube. If you are using a name brand Rubik's Cube, each square is 16mm x 16mm. I then made a row template 16mm x 48mm (48mm comes from multiplying the the 16 by 3 since there are 3 squares across and down.) Then I picked out 6 pictures for the cube and cut them into a 48mm x 48 mm square.
Next, turn the picture face down and draw out a grid like a tic-tac-toe board using the 16mm x 48mm template.
Cut out the little squares and glue them onto the cube. I used a sticker maker to do this part but a glue stick will work just as well. My cutting was a little off but no big deal.
After all the picture squares are on the cube, use modge podge, or something like that to keep the stickers from peeling up. After sealing each side of the cube, let it dry. Once dried, turn each section a few times to break up any glue residue.