Put a mark just below the roof line on the side of your home you want the water to run away from.
Measure the length of the wall that the gutters are going on and divide that length by 10 feet. Round the number you arrived at up to the next whole number.
Multiply your whole number by 1/4 inch and write that number down.
Attach a line level to a string and run the string from one end of the house to the other with some help from a partner. Hold your end on the mark you made and have your partner make a mark where the line says is level.
Measure down from the mark by the value that you wrote down earlier and make a new mark. Run a chalk box from your original mark at the other end of the house to the second mark that was just made at the opposing side. The second mark is where your downspout needs to be installed because the gutters need to slope down toward the downspout for effective water removal. You now have a guideline for your gutters to follow.
Cut the final gutter of your run to the proper size so that it does not run past your home. Make a mark around the gutter using a pencil and a combination square. Cut the line using heavy tin snips. Attach brackets within six inches of either end of each of the gutters and every 18 inches throughout all of the gutters.
Screw the brackets into the side of your home, locking the gutters in place. Push the top of your downspout against the bottom of the gutter you want to attach it to and trace one edge with a pencil. Take the last gutter back down and mark and place one edge of the bottom of the downspout sleeve against the pencil mark you made for the edge of the downspout. Trace around the sleeve and cut it out using a jig saw. File the sharp edges smooth.
Put silicone caulk around the underside of the flange on the downspout connector. Place the connector into the gutter from the top and screw the flanges down in to the gutter.
Place an end cap over the end of the last gutter and crimp it in to place. Attach the aluminum mounting straps for the downspout along the wall the downspout is going to be attached to. Set the downspout in place over the connector and screw the aluminum straps to the front of the downspout, locking it in place.
Attach small seamer strips of gutter over all of the joints between gutter pieces and crimp them in place. Apply a thick bead of silicone caulk over the joint on the inside of the gutter to make it water tight. Allow the gutters to dry for a day and pour water in them to test for water tightness.