I prefer to make a dirndl skirt from the flat pattern for a basic skirt because I have more control over how much width will be added for gathers in the front and the back. There is more control afforded because you know exactly where side seams of bodice and skirt should meet. You can also vary the amount of gathering if you wish. Here you see the front skirt pattern after it is drafted with 1″ of style ease added for the waist and 2″ of style ease added for the hips.
Vertical lines are drawn through the darts to the hem of the skirt. Then on either side I added additional lines. These are then cut open and spread the amount desired. I draw a straight line at the hip level and match the hipline from the pattern upon it when spreading the pieces apart. I usually find 1-2″ enough for the gathers. For The Dirndl Dress I opened all slashes in front and back 1″ each.
After spreading and taping the sections, the pattern is cut straight across the waistline, eliminating the upper part at the side seam.
The same steps are taken with the back pattern except that since the lower center back skirt length begins lower than the front, the line from side seams to center back retains the slight angle it would in the basic skirt pattern. This ensures less bunchiness at the back.
Important Note: The L-square ruler is used to even out the side seams of front and back skirt pattern piece after cutting and spreading for the gathers. The ruler should be placed in line with the side seam of the skirt and drawn straight up so that the hip curve is no longer used as part of the side seam. This results in slightly more width but not that much.