installing a rain gutter do you ....?

The gutter systems I have used need the hardware to hold the whole thing together.

I had the same problem, so I bought L shaped flashing, slid it under the shingles above the fascia and over the lip of the gutter. That worked for me.

some answers just should not be posted!!!

If you were to nail the gutter you do not allow for expansion ( even in plastic!) You need to put the hardware up first. If you say that the rain is getting in between the facia board and the gutter then I suspect you are trying to butt up against a roofing which is probably felt or similar which has been moulded into position by heat and then sealed, not intending to have guttering attached (otherwise any form of tiling or slate will have an overhang beyound the facia.) If that is the case you need to tease the overlapping felt away from the facia and into the gutter, using extra felt underneath if required.

If I was you, I'd nail it to the facia board.