The first step in installing a gutter system is to make reference lines to show you where the gutters should be attached to the eaves. A proper working gutter system should be slightly sloped to allow for drainage to the downspouts. A good rule of thumb to use is that the gutters should slope down a quarter of an inch for every ten linear feet. Start first by attaching a line leveler to a chalk line and mark a level line. Then measure down a quarter of an inch every ten feet and make a new chalk line connecting those marks. This is the line on which the gutters will be installed. You will find, especially with older homes, that there has been some settling and that the roof lines are not level. If that is the case you can use the slope to your advantage when installing your gutters. That is why it is important to mark all of your lines before you make any cuts or hang anything. You might find you need to change the location of your downspouts to take advantage of a natural slope of the roof line.
The next step is to take accurate measurements of all the areas where you will be installing gutters. Make sure to account for mitres or elbow and seams. Hold up the corner pieces and mark how far they come in from any turn. Then allow a quarter of an inch for the seams. The remaining space is how long the straight pieces need to be. After you have checked and rechecked your measurements, you can cut the gutter pieces. Some how to guides suggest that you use a miter saw to cut the gutter pieces, but if you are using aluminum or vinyl gutters then you can get the job done with a heavy duty pair of tin clips.
The third step is to install the brackets and mount the gutters on top the eaves. Follow the manufacturers recommendations as to how often and how far from the edges you should install brackets. Make sure to make marks on the gutter pieces and on the eaves and then match them up accordingly. You will need a power drill to screw the brackets into the eaves.
Before you hang the pieces of gutter that will contain the downspouts, you need to make a mark where the downspout will go and then cut the holes. Once you have pencil marks showing where the downspouts will be use an attachment sleeve as a guide to trace an outline for the rectangular hole. You can punch four small holes in each corner with a crew driver and then enlarge the holes with a power drill and bit. Use a jigsaw to cut out the rest of the hole and then file down the edges. You should then install the downspout attachment by placing a layer of silicone sealant on the attachment and dropping it into the hole you cut in the gutter piece. Then screw the attachment into place with short screws.
When you need to cut pieces of downspout, make sure you pay attention to which end you are cutting. One end of the downspout is tapered so that it can fit into another piece without the aid of a connector. You always want to cut the large end so that the tapered end is still available for connecting other pieces. Another helpful tip is to always connect the pieces so that the large end is facing up and the tapered end is down. This will prevent leaking at those seams. Downspouts are attached to the side of the house with mounting straps. The straps get screwed into the side of the house and then wrap around the downspout to keep it in place.
The next to last step is to connect the straight pieces of gutter to each other and the corner pieces. Remember that there is a quarter inch of free space between all of these pieces. The solution is a two inch wide strip of aluminum or vinyl called a sealer. The sealer wraps around the gutter pieces and covers up the quarter inch gaps. The seals are made water proof by applying a heavy bead of silicone sealant on the inside.
After waiting for the sealant to dry, the last step is to test your gutter system by putting a garden hose in one end and making sure that the water flows freely from one end to another and to the downspout. If there are any areas where water pools you will need to adjust the pitch of your gutters to correct that. If there are any leaks, then reapply your sealant. If you want to limit the amount of gutter cleaning you will need, then you might also consider installing leaf guards or a more complicated gutter guard system.