Originally Posted by quilticing
Use a long ruler to extend the angle of the side seam. I use tissue paper or newsprint (roll-ends) taped to the pattern piece. Draw the line. Then measure equal distances from the bottom (hem) edge. Connect the dots. It will probably be a slightly curved line. Don't forget to add for the hem allowance.
That would work well if you want just to extend the hemline down to make it longer.
If you want the "skirt" part to look separate from the "top", you could cut a piece of fabric (grainline up and down):
the extra length you need + 2"
by
1 1/2 times the circumference of the shirt bottom.
Stitch the side seam together so you have a tube.
With the longest stitch on your machine, run a basting line 1/2" from one long edge (this will be the top), beginning and ending just before the seam. Leave the threads long on both ends. Mark the tube into 4 (or more) sections with pins or chalk. Do the same with the shirt portion. Pin the marks on the two pieces together, right sides together. Pull up the threads, moving the fabric along, until the sizes match. This will give it a little bit of a ruffled look. If it looks too ruffly when you pin it, make the tube slightly smaller and try again until you're happy with it.
Stitch a 5/8" seam, finish the seam as you like, and hem the bottom.