So I tried repairing my older, rotted sheathing near the eave fascia of my shed. I re shingled about 6 rows and now there’s a leak inside coming from where the new sheathing meets the old sheathing. This leak is happening near the top two rows of new shingles I laid. Near where the new shingles meet the old shingles.

My guesses:

It’s coming from a few nail holes I may have missed in the row of old shingles where I had to take nails out to slide the new shingles in.

I am really not sure where to start because I don’t know if the leak is coming from below or above it. Since it’s a low slope roof

Where do I start to find this leak?