Carefully remove the nails out of the bottom row of shingles with a flat bar.
No need to do it also with the starter shingle.
Remove the seal of the shingle with a sliding, Chipping motion with the flat bar.
Once you have broken the seal of the shingle
Get the flat bar under the nail and wiggle it up.
Now you can slide your flashing under it.
I would use a flexible flashing
Like MFM self-adhered modified bitumen.
White or aluminum 12 inch wide roll if available possible.
It would be the best.
You could also use any granulated
Self-adhered modified material available to you.
But i am suggesting the best.
Might have to cut it out of a 36 inch roll if you just cant get what i have suggested.
Slide under main roof material
Under the nails you have just removed.
Get it set all in place.
Now Peel the backing off.
Form it to the corrugated metal slowly and carefully.
Relax. Stay calm.
Re-nail the shingles.
You also need to reseal the old shingle seals you broke with
Also Re seal the old holes in the above shingles you removed the nails from.
Put some paint or stain or water proofing paint on the fence.
It is much easier. Please.