Use some other tape as a guide.
Trim around your new decals and with the backing paper still on lay it on the rim. Use something like the Scotch "blue" painters tape, that's easily removed, to mark where your decals edges are when you have the decal lined up.
If you still have the old decals on the rim, mark the position before removing.
If it's a big decal like those on a Zipp 404 or deeper (or a disk) or on any all carbon wheel, pre-marking is very handy to ensure that you don't end up sticking the decal onto the braking surface. I know first hand that the adhesive on Zipp decals is very strong - you need to get them on correctly first time.