Do what the real roofers do. Cut the shingle strips into individual tabs and lay the tabs cross wise over the peak, overlapping them a bit. You don't have to cut off the 'hidden part of the tabs like the prototype does. Tedious work but it will produce realistic results. I would clamp a straight edge of some sort to the roof to keep the ridge cap properly aligned, or draw a very faint pencil line.
I would also suggest that you may have to add another row of shingles at the top of each peak. You will have to cut a portion of the 'hidden' part of the shingle strips off where it sticks up above the roof, but I would do that after the shingle glue has dried. You may not see much of that last row of tabs but by looking at your pictures I don't think the top row of shingles is close enough to the ridge for the ridge cap tabs to cover properly. You will be able to tell if you need another row simply by putting a ridge tab temporarily in place to see if it covers all of the part of the shingle strips that are supposed to be hidden.
Dave
(20 years in the real roofing business).
By the way, your valleys are going to leak badly! Next time put a strip of paper down the valleys first to simulate metal valley material, paint it, and then cut the shingles slightly short of the center of the valley so the valley material shows a bit. Look at the picture Rob posted.
