Outside Corners: To start a corner, nail up the first shingle so that it overhangs the corner by at least an inch. Use extra nails at both the tip and the butt to prevent this first shingle from moving or pivoting during trimming.
Trim the excess off with a sharp utility knife or the blade of a shingling hatchet to within about 1/2 inch of the finished size.
The second shingle should cover the edge of the first, again overlapping the edge by about 1 inch. Trim this shingle too with a utility knife and finish it with a block plane, taking care not to scar the face of the first shingle as you plane the second shingle flush. To finish the joint, drive a 4-penny nail into the edge of the first shingle approximately 1 inch up from the butt and 1/8 inch in from the edge. This will help prevent the shingles from curling and opening up at the joint.
Inside Corners: One common way to finish inside corners is to butt the shingles to 1 1/8-inch by 1 1/8-inch cedar trim, but they can also be woven. To do this, trim material off the bottom of the edge of the shingles (rather than from the top of the edge, as is done with outside corners). After installing the first inside corner shingle, butt the second one on the facing wall against the first and level it with a small torpedo level.
Just a few strokes with a block plane along the inside edge are usually sufficient to get a tight fit.