-Remove the old ones. Putting the plastic in the freezer for an hour or so seems to help a little.
-Clean the plastic to get old goo, dirt, oils etc... off. Goo-Gone is good for removing the old adhesive but be sure to get it ALL gone prior to installing the new graphic!
-I wash my plasic with dish soap to be sure the Goo-Gone is gone.
-I wipe my plastic down with rubbing alcohol after it is "clean".
-I warm the graphic a little. I used my wife's blow dryer but when I burned the last one up doing that I bought a new blower dryer for her AND a heat gun for me. Good thing Target is open late because she needed the new one to get ready for work the next morning.
-I put some rubbing alcohol in a spray bottle and give the plastic and the back of the graphic a light mist.
-Then position the graphic on the plastic starting at the largest, flattest & straightest edge (Usually where the shroud goes under the seat.) and work it across from there. Some instructions say to start in the "middle" but that has ALWAYS caused me grief and bubbles.
-A small rubber squeegie and some of that alcohol sprayed on the top of the graphic helps smooth them on the plastic.
-Work slowly.
-The heat gun/blow dryer can help make getting the compound curves smoothed.
-Keep working slowly.
-A needle might be needed to remove some of the bubbles that you worked on to the plastic slowly.
-Let the grahics sit for a day.
-Go out and mess up your new graphics.
I actually placed my last set of graphics on with the plastic on the bike and it gave me a much nicer (i.e. smoother) job because the plastic was in it's final position.