the best way to install these is the following: (trust me I've installed more 3m taping accessories I have it down to an art)

once you have wiped the surface with an alcohol swab (clean the dirt and dry any moister) you fold 1 small corner of the 3M tape so that the peeled part is facing towrds you rather than in the direction of the length of the sill. You should have a small peice exposed to allow to stick or remove to set the sills in place, once you have it set slowly pull the tape towards you and sideways along the sill, this will allow you to remove the tape easily while the sill stays in place. if the sill has 2 strips just repeat for teh top strip but fold it away from you and pull that direction.

If there was a more clear way for me to describe it I would, I will be installing mine on the weekend so I can take some pics and show you what I mean.