How do I adjust the crotch curve on trousers?

Treat crotch depth and crotch length as two different adjustments, because the drag lines they cause look similar and the fixes are not.

Depth first. Measure sitting on a hard chair from the side waist to the seat. Compare that with the pattern, measured from the waist seam line down to the crotch point on the side. Adjust at the lengthen and shorten line above the crotch, on both front and back, so the crotch point sits where the body’s does. Wrinkles under the seat and a crotch that hangs low are depth problems.

Then length, which is the curve itself. The front and back curves control how much fabric passes between the legs. Horizontal wrinkles pulling at the front usually mean the front curve is too shallow: scoop it out by a few millimetres, redrawing the curve smoothly from the inseam. Fabric catching under the seat usually means the back curve needs scooping or the back rise needs extending.

Change one thing at a time and by small amounts. Three to five millimetres on a crotch curve is a visible difference on the body.

Scooping adds room; adding to the curve removes it. Scooping also lengthens the inseam slightly, so walk the inseams front to back afterwards and true them.

Do this on a toile in a fabric of similar weight, with the waistline held at the correct level by elastic. Trousers fit is the hardest fitting in dressmaking and it is not solvable by measurement alone.