Don't use the thick foam type double sided tape, as the badge will stick out from the body and wont sit flush.
In the past, I have used the thin 2 inch wide tape ("Bear" brand) . You can get it from Autobarn.
I put the tape over the badge and with a small blade knife, trimmed the excess off.
It's a bit fiddly. Just take your time.
Another method I have used is Silastic.
I have just applied a very thin film of it to the badge and held it in position with some masking tape. The only down side to this method is that you will have to allow the recommended drying time.
If you are mounting a badge without locating pins, this method allows you adjust it, unlike double sided tape which has instant adhesion.
Just clean up any excess with Methylated spirits and a cotton bud. The Metho is excellent for cleaning up Silastic.
I've also found that if you have to remove the badges, the silastic is really no harder to remove from the car than the double sided tape.
Remember though, ONLY use a very thin film.