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.
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