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.
Next, cut off the sleeves, they can be pockets on the finished garment if you want. The first cut you will make is the front crotch, cutting while the t-shirt is folded in half (this guarantees that the front and back crotch will line up, as the inseams need to be the same length). This measurement is the distance from the bare floor to your preferred waist sitting down. If your crotch measurement is 11”, start your cut 11.5” below the neck line. Make this a shallow curve, as you can deepen it later. You will cut the waist angle when you open the shirt back up.
Next cut the inseam from the crotch to the hem. I recommend using the circumference of you knee bent for the hem width, divide this in half, and mark from the fold to the bottom of the inseam. I like cutting from the hem to the crotch, cutting straight or curving a little as you get to the top (but make sure you end up with a straight angle at the top). This is easier than cutting from the top down.
Next open the t-shirt up and cut the waist angle, as straight as possible, but higher in the back than the front. You can adjust this later, but what you are after is a straight angle at the top of the front and back crotch curves, so you start by getting a straight angle at the front top crotch line, cutting as close to the neck line as possible. As of this step, you have not cut the back crotch yet (it is still the shape of the front curve). So now it is time to adjust your pattern.
Measure thy width at the widest point, making sure it is 1” wider than the loose but fitted measurement of the top of your thy for seam allowance (very small) and ease. Adjust by folding in half again and cutting both sides together.
Now measure waist, divide by two and add seam allowance. Adjust your crotch curves to meet up to your waist measurement, again, making sure the cures are at right angles at the top and bottom of the curve. Figures that carry more weight in front will want a curve that is more like the back of most pant patterns, so this is really where the customization of the pant resides, in the shaping of these two curves).
Baste you pattern t-shirt together before cutting you sample garment. With right sides together, sew front and back crotch seams. Then fold front and back crotches right sides together and stitch the length of both leg’s inseams. Try on inside out, so you can pin the adjustments you want to make to the pattern.
For your final garment, cut a piece of elastic for the waistband that is smaller than the waist of the garment (and comfortable around your waist), and zigzag raw edges together into a circle. Mark or pin the side seams of your pants, and the front, back and side seams of your elastic, and fold the waist band over the elastic matching pins or marks, stretch, and zigzag in place! Hem bottom of pants, or leave raw. Enjoy!