Begin laying the second shingle panel layer in the roof's lower left-hand corner and work your way up and across the roof in a triangular pattern. Slide the first new shingle panel's top edge up tight against the existing shingle panel's bottom edge.
Nail the new shingle panel in place with roofing nails and a pneumatic nailer. Use four to six nails per shingle panel. Do not place nails through the portion of the shingle that will be exposed.
Lay another shingle on top of the shingle you just laid. Butt this shingle's top edge against the existing shingle's bottom edge. Slide this shingle panel down halfway and center its first full shingle over the space where two shingles meet on the first panel. Secure the second shingle in place with roofing nails.
Cut a shingle panel in half with a utility knife to fill the gap left by the second shingle panel. Lay this cut panel in the gap with its right edge pressed against the second shingle's left edge. Push the cut panel's top edge against the existing shingle panel's bottom edge and nail it in place.
Place a third shingle panel on the first shingle panel's right-hand side. Press the two shingle panels together and butt the third shingle panel's top edge against the existing shingle panel's bottom edge. Secure the third shingle panel in place with roofing nails.
Continue to work your way across the roof in a triangular pattern until you cover the entire roof with a new layer of shingles. Use a tin snips to cut the shingles so they extend just 1/2-inch past the roof.