I used the 3m tape (from the local auto parts shop) as well to install my Aero Lip. This keeps it secure, but yet, it makes it easy to remove it if you ever damage it (like hitting a curb like i did!
When i did remove it, there were no markings on the bumper where the aero lip was. So I dont think you need to worry about scratching whats underneath it.
As far as how much tape to use, if you look at the underside of the lip, there are some recesses for the tape placement. Just follow those recesses and you'll be fine.