Roofing Valley DIY Question

This DIY roofer has to put a 20-inch flashing under a 10-foot valley,
because the original (professional) roofers put a lot of nails in the
valley, and the nails are sure-enough starting to leak just like the book
says.
What will go wrong when I try to pry up the shingles (3-foot 3-tab
fiberglass) and push the big galvanized metal under them. Should I just
remove shingles, or try to lift them a little and push the flashing under
them?
My flashing is flat, not a "W" shape, so I plan to tar the whole flashing
to the shingles it touches.