Measure along the total length of the wall where you are installing gutters. Divide the distance of your barn wall by ten feet and round your answer up to the next whole number. Multiply this number by a quarter of an inch and write the answer down for future reference.
Pick the side of the barn that you want the water to run away from and make a mark just below the roof line. Position a partner at the other end of the barn wall. Attach a string level over a string and run it from one end of the barn to the other.
Hold your end of the string on the mark you made and have your partner hold his end at the spot that reads level. Have him mark the location. Measure down from there by the amount you have written down. Make a new mark there and erase the first.
Run a chalk line from the mark at one side of the barn to the mark at the other and snap a line. This is your gutter guide.
Subtract the length of one gutter from the length of the wall until you are left with a number less a gutter. Cut your final gutter to this length and crimp an end cap to the end of it that is not going to be connected to any other gutters.
Install screwing brackets within five inches of either end of the gutters. Install brackets throughout the gutters making sure that there are no more than 24 inches between any two brackets.
With help from a partner hold each of the gutters up with the top edge in line with your guideline. Drill the screws in the brackets into the side of the barn, locking the gutters in place.
Hold the last gutter up against the wall and have someone else hold the downspout up so that it is touching the bottom of the gutter. Mark a line along two of the downspout edges on the bottom of the gutter.
Place your downspout connector between the two marks you made on the bottom of the last gutter. Trace around the connector and cut it out with a jigsaw. File the rough edges of the cut.
Put caulk around the underside of the flange of the connector and insert the connector in to the gutter from the top down. Screw the connector in to the gutter. Install the last gutter and crimp the seam pieces over all of the seams between the gutters. Apply a thick bead of sealant over the seams on the inside of the gutter.
Attach the aluminum straps along the wall where the downspout is going to be connected. Set the downspout in place and wrap the straps around it. Screw the straps to the front of the downspout to complete your gutter installation.