This is an easy one, and it is the first thing to do before fitting a pattern. I use this method for all patterns (see my previous mini-rant on stitching lines not existing in any modern patterns)
- french curve (optional)
- pen of a easily-identifiable color
Most modern patterns have a 5/8" (1.5cm) seam allowance. However, make sure you check the correct seam allowance every time.
Steps: The idea is very simple: mark the seam allowance length from the cutting line. The marked line will give you the stitching line.
Step 1: Find the 1.5cm (5/8"in) measurement on your ruler. Mine is shown on this picture:
Short tip: the width of a tape measure is usually 5/8". Now, I own two tape measures: a plain one and a fancy one. The fancy one is slightly bigger than it should be, but the plain one is the expected 5/8".
Step 2: Identify the cutting line for your size. I'm cutting a size 2, so I have the dotted line in the picture above.
Step 3: Start marking the stitching line with short lines. If you can do dots, that's even better, especially for curves.
Make sure that each mark you draw is parallel to the original cutting line.
For straight pattern lines, you can just draw the line directly:
Once you're done, the pattern will look something similar to this:
Step 4: Take your french curve and find a curve on it that matches the dotted lines.
Alternatively, you can just freehand the line. Or skip this step altogether.
Extra: you can also mark the notches. I mark the relevant notch, then draw a line perpendicular to the cutting line, from the cutting line, through the vertex of the notch through to the stitching line.