I put my Sockmonkey decals on this morning. I used blue masking tape to help align the sticker (it's easier to move the tape and measure than it is to move the sticker). I positioned it so that the pull-back paper of the new sticker butted up against the tape's edge. I also lined up the left edge of the tape so that when I put the corner of the decal there, it was at the same position as the stock sticker.
Front passenger side panel: