I drafted the pattern for these by using my pencil skirt block, and making a few alterations, and decided to share the process with you all. For the sake of clarity, I did new photos using a mini-pattern to demonstrate.
You will need:
Cloth tape measure
Traced copy of a pencil skirt pattern or fitted skirt block, with at least 1 pair of darts on front and back.
Your base pattern should have little or no ease drafted in, and no seam allowances. If starting from scratch, you can simply draft a skirt block without ease or seam allowances. If using an existing pattern, make sure to trim away any seam allowances. My pattern had 1" of ease at the waist, and 2" at the hip line. To remove this, I folded under 1/4" along the centre front and centre back lines, then trimmed the side seams by 1/4" at the hip line, tapering up to nothing at the waist. If you are unsure how much ease you have built in to your pattern, use it as is and make a toile to remove the ease at the fitting stage. My pattern had one pair of darts on the front, and two pairs on the back: the darts are used for adding shaping to the pattern pieces later.
My base pattern:
Marking front and back lengths
The first step in converting your pattern is to mark the lengths. For this, you will first need to measure your crotch length.
Find your waistline by tying a length of elastic snugly around your middle. Move around, bend from side to side; this will settle the elastic on your true waist. Take a tape measure, and measure in a U shape from your waistline at the front, down between your legs and back up to the waist at the back, holding the tape quite snug to the body. For reference, my measurement here was 30".
Divide this number in half: so for me, 30/2 = 15".
On the front pattern piece, measure down your half crotch length minus 2" at the centre line. This would be 13" on mine.
On the back pattern piece, measure down half your crotch length minus 1" at the centre line, so 14" on mine.
Next, measure down your side from the waistline to where you want the hem of the knickers to sit. I went with 9". Mark this on your side edges on both pattern pieces.
Marking the leg openings
Draw in the curved hem edge for the front. The curve should be quite wide and shallow for some distance from the side, curving more sharply down towards the gusset. Continue the curve into the gusset area, beginning to curve back out again. As you can see, I took a few attempts before finding a good line.
Next, draw in the ends of the gusset, curving them outwards . In the image below, the left end of the gusset will join to the knicker front, and the right end to the knicker back.
Now cut the gusset piece out, following the curved lines. This will leave an inward curve at the bottom of the front body piece. Line the gusset up with the crotch length mark of the back piece, and trace the curved seam line on. Draw in the hem line of the back piece. I curved outwards on this piece for extra coverage.
Cutting the panels
You are now ready to cut the pattern into panels. First cut along the curved hem lines we just drafted, and discard the lower parts of the skirt pattern. Your pieces should look similar to this:
This is where the darts come into play. On the front piece, extend the forward leg of the dart down towards the hem in a straight line. Cut along this line. Cut away the inside of the dart and discard.
These two pieces will become your front, and part of the side panels. Repeat the process for the back in mirror image, as follows. If your pattern has two darts, like mine, you will cut the dart closest to the side. Extend the rearward leg of the dart towards the hem, and cut as for the front.
If your pattern only has one dart, the back piece needs no more alteration. However, if you have a second dart, it needs closing. Cut away the inside of the dart, and slash the pattern from the point of the dart to the side seam in a couple of places. This is how I cut mine:
Tracing the final pattern
Take the slashed back piece. Rotate the parts so that the dart is closed, and the edge has a smooth line. Trace the piece, rounding out the curved edge over the spread sections.
NOTE: Spreading the pattern like this creates a little extra length along the seam edge. We will need to adjust the side piece in the next step.
For the side panels, there are two options: keep them as two pieces with a side seam, or merge them into a single side panel. I did the latter, as my side seam wasn't especially curved. Overlap the two side seam edges, matching the top and bottom corners quite closely. Trace around the two pieces as one, smoothing out the curves.
If you slashed and spread the back, you now need to amend the length of the side panel. Line up the side-back piece you cut away with the spread and traced back piece. Start at the waistline, and check the length down towards the hem edge: the side piece will be short.
Measure how much extra length is needed, and add this onto the corresponding line on the traced side panel. Draw in a new hem line.
Trace the front piece, and the gusset piece, flipping the gusset down its centre line to make a whole pattern piece. Mark the front and back edges of the gusset: I added notches to the front edge as well as "F" and "B" on the relevant ends. Add your preferred seam allowance to all edges, and cut out.
The completed pattern!
I highly advise making a toile to test your pattern before using any precious materials. My leopard print ones were made from a scrap of fairly low-stretch single jersey left over from a dress and some 9" wide embroidered stretch tulle that I have a large length of. Sew up the major seams and try on for fit. Mine needed to hem line raising about an inch on the back piece, and taking in a half inch on the two back seams. The fit will vary widely depending on the fabric you use: those with more stretch will likely need to fit more snugly, while a less stretchy fabric like my single jersey may benefit from being looser fitting.
I finished the edges with picot elastic at the waist, and some narrow stretch lace around the leg openings on the jersey panels, leaving the shaped edges of the tulle as they were.
I'll be making up a couple more pairs in different fabrics this week to match my two lace camisoles from the previous post, so I will report back with more pattern testing soon!