How do I add seam allowance to a pattern?

Draw a second line outside the stitching line, parallel to it, at whatever allowance you have chosen — and do it before you cut, not while cutting.

The tools that make it quick. A double tracing wheel, a seam allowance ruler with a fixed offset, or a simple compass. Failing those, mark a series of dots at the chosen distance perpendicular to the line every couple of centimetres and join them. On a curve the marks must be perpendicular to the curve at each point, which is why freehand offsets drift.

Choose the width by location. Around 10 to 15 mm on straight garment seams. Around 6 mm on tight curves such as necklines and armholes, because a wide allowance on a curve has to be clipped anyway. A deeper allowance — 20 to 40 mm — at hems. More on any seam you expect to let out.

True the corners. Where two allowances meet at a corner the outer lines cross past each other. Extend both and cut to the crossing point, so the allowance folds correctly when sewn. At a dart, fold the dart closed as it will be sewn and cut the edge through the folded paper, which gives the correct stepped shape.

Mark the stitching line as well, at least at the start, so you can check the machine guide against it on a test seam.

Write the allowance on every piece. A set of pieces cut with mixed allowances is unusable, and the mistake only shows up when the pieces will not match.