First, start by noting the following measurements on a piece of paper:
- Circumference of the top of your thy(s),
- Crotch depth (measured from bare floor to where you want the waist of your pant to sit,
- Widest part of hips, and waist (where your pants will sit).
You will need two t-shirts that are wider than the circumference of the top of your thy, measured from under arm to underarm, across the width of the t-shirt. One will be your pattern, the other your sample garment.
Cut off the top of the pattern tee at a slight angle, directly under (and as close as possible to) the neckline of the tee. You want this angle to be an inch or two higher at the end of the cut than when you started (or more, depending on how much room you need in the rear). Now cut the width of the t-shirt to your thy measurement + seam allowance. Fold the t-shirt in half, matching side seams, with shortest angle on top, and smoothing so that there are no hidden folds. Using your crotch measurement from the shortest angle of the 'waist' cut, cut through the four layers of the tee, creating a shallow curve and straighten out toward the top. You will create a deeper curve for the back using the front crotch as a guide, so keep this curve shallow to begin with. You can cut deeper cuts as you adjust, but you cannot add to fabric already cut.
To guide you for the angle you need for the back of the pants, unfold the tee and measure half your waist (30" waist is 15") from the front crotch cut and place a pin. Stretch this to make sure the waist will fit over your hips (has to stretch to 20" + seam allowance for 40" hips). This is your guideline for the back crotch curve (and for adjusting the front curve). Your back crotch curve will be deeper and curved at an able that ends in a straight angle at the back top of waist. This curve will depend on your body type. If you carry more weight in your waist than your seat, the curve will be shallower than if this is reversed. This is the 'wiggle room' you will need to adjust as you perfect your pattern.
You should baste stitch your pattern together to see how close you are before cutting out your sample garment. Sew right sides of front and back crotch seams together, then sew inseams front to back, right sides together (there are no side seams in this design). Make adjustments, etc., including tailoring inseam and bottom/length of your pant to your liking. When cutting the inseam, fold in half matching crotches, and cut all 4 pieces together.
Cut a piece of elastic for the waistband that is smaller than the waist of the garment (and comfortable), and fold into the top of your sample garment, matching front/back and side seams of garment and elastic. Zig zag in place! Hem bottom of pants, or leave raw. Enjoy!