Take a section of guttering and mount it on a bracket.
Position it against one end of the fascia board as close to the roof tiles as possible.
Using a pencil or marker, mark the top of the bracket.
Using a nail, attach a piece of string, long enough to reach the opposite end of the fascia.
Move to the other end of the fascia and tighten the string, use a spirit level to ensure it is positioned horizontally. Secure the end of the string with a nail.
Repeat this process for all the fascias.
Using a plumb line, mark positions on the fascia which are vertically above the rainwater drainage points on the ground. These are the points at which the gutter will be lowest.
The gradient of the gutter will need to be at least 1:600 (i.e. for every 3m of guttering, a 5mm fall is needed).
If there is one outlet at the end of the fascia, calculate the fall needed for the length and adjust the string as required at the outlet end.
If there are outlets at each end of the facia, add an additional nail, to hold the string, in the middle of the facia. After calculating the necessary fall for each side, from the middle of the facia to the end; lower each end of the string accordingly.
If the outlet is part of the way along a fascia, lower the string at this point, calculating the fall using the longer length of facia, attach a nail to support the string.