A drip edge and a gutter both remove water from the roof of a house. Ideally they work together to do that job. Drip edge is a metal strip installed between the roof decking and the shingle to prevent water from getting under the edge of the shingle and onto the wood. Guttering runs along the roof line to collect water that runs off and drain it off through a downspout to some area where it will not create problems. A properly installed drip edge will funnel that runoff into the gutter.
Use a ladder and a tape measure to measure the roof line where drip edge is needed; ideally install drip edge during construction and before gutters are added, but it can be done later. Decide on a style of drip edge. All drip edge has a 90-degree angle so one face fits under the shingle and the other points down over the facing board of the roof, where the gutter is installed. Choose a type with a downward angle that is wide enough and slopes out enough to guide water into the gutter.
Install drip edge with short nails if no shingles are present; if shingles are already installed, use roofing cement under the top plate of the drip edge, sliding the roof angle under the shingle and pressing down to secure the edge. Test the downward slope with a piece of gutter, or if guttering is already installed make sure the drip edge will direct water from the roof into the gutter. Use a drip edge that has a slight outward bend at the bottom to direct the water; the alternative is to bend the drip edge slightly outward to the correct drain angle. Test it by pouring water on the roof and watching it flow to the gutter.
Mitre or overlap drip edge at the corners of a roof. Cut 45-degree angles with tin snips to mitre corners, so the point of the mitre is at the corner of the roof. Overlap by cutting a notch out of the top strip the width of the piece being overlapped and folding the corner side around the roof edge. If the drip edge is 1 1/2 inches wide on both faces, cut 1 1/2 inches from the top of the piece on the side, nail that on top of the bottom piece, then fold the remaining section of the side piece around the corner. Make sure this overlap flows into the end of the gutter at that end of the roof. Install guttering, if not already present, under the bottom of the drip edge.