Here's an example of how to add seam allowance to an a-line skirt. The hem gets wider so we will need to check how it will be folded before we can add hem allowance. Align the center front with the folded edge of the fabric. If you're re-drawing the pattern on paper align with paper edge.
1. Add seam allowance to all other edges.
2. Add a notch where hem allowance begins. This marks the fold line for the hem. Transfer any other notches and pattern markings.
3. Add hem allowance. Here, we're adding 4 cm (1 1/2").
4. Pivot the ruler, following the curve.
5. Flip down the pattern, matching the outer corner with the notch and making sure that the middle front edge is lined up with the fold line. Add seam allowance along side seam until the line meets hem line.
6. Remove the pattern. Cut along the lines, making sure to cut away all tracing lines.
If the hem is very wide, such as in a circle skirt, it's easier to add a minimal amount of hem allowance and finish it with a rolled hem. Another option is drafting a hem facing instead.