We have all put a sticker or decal on crooked but if you don't know this little trick it might help alleviate future mistakes. A sign-writer taught me this trick a million years ago and you will notice its'use on many things. If you have a decal on a sheet backing, first carefully peel 1/2 the length of the decal off the backing (front of decal flat on table,- hold decal down with thin modeling knife or similar if necessary- NOT FINGERS) while holding backing up and un-peeled side down with left hand , cut lifted backing in half (near where it's still attached, then carefully re-apply to decal (same non stick side down). Draw a faint SOFT pencil line where you want the decal on the model (or whatever) for the lower edge of the decal and the left hand start of the decal. Line up the decal and hold down the original unpeeled side firmly. Flip the previously peeled (and re-stuck) side back and start peeling the backing again from the cut while gently rolling the decal on as you peel, holding the right hand end . When almost at the end, completely remove backing and smooth decal from center out with a folded handkerchief or similar. Do the same thing with the original side and now your decal should be dead straight. Practice makes perfect.