How Do I Get the Correct Angle on My Rain Gutters?

Rain gutters should slope between 1/3 inch and 3/4 inch per 10'
towards the drain spout.

Were you asking for the slope or are you asking how to achieve the slope?

To achieve the slope: 
Lay out the pieces of gutter and fittings on the ground below where
they are to be installed. Align them to correspond to the way they
will fit when assembled under the eave of the roof. Use a long level
to get the correct slope for each run of gutter. A slope of
approximately 1in. for each 16 ft. of gutter is adequate to provide
proper drainage. Good drainage is important.

One easy way to accurately set the slope for proper drainage of gutter
is to allow for a fall of 1-1/4 in. for each two 10 ft. lengths of
guttering material.

This can be calculated by marking the nailing position on the fascia
of the house before attaching the gutter and then checking the fall
with a level to be sure it is accurate.

Locate the center of each span of gutter material. Mark this center
location on the fascia of the house. Now take a chalk line and snap a
line from the center position to the end of the run in each direction.
Allow for 1 in. of fall each way. On an ordinary home installation,
this fall of 1 in. in each direction from the center will provide
adequate drainage. If the run is extremely long, allow 1 in. fall for
each 16 ft. of gutter.

Search Strategy: Google how to slope a gutter
Portions of above answer from: http://www.dulley.com/diy/df106.htm

Also: Good instructions with images here:
http://www.doityourself.com/info/installnewgutters.htm