A couple of years ago I made a pants sloper. I learned, during that process - which, while painful, was in NO way even half as challenging as making a standard woven top (for me, given my body type) - that I don't need to shorten the legs of pants (my legs are proportionately long) but the rise of pants.
You see, my lower torso is very short and, between my navel and base of my crotch, I am a couple of inches shorter than the length of your average commercial pattern. So, these days, when I cut out a new pants pattern, I always compare the crotch length to that of my sloper. Inevitably I remove the same 2 inches at the lengthen-shorten line. Not a prob.
My crotch curve is also not standard - odds are that yours isn't either. I need to add a bit of fabric onto the front and back crotch seams to give a small amount of extra room given that a) I need to accommodate the bump of my lower abdomen and b) I have a relatively round derriere. Having said this, my legs and hips are proportionately slender.
Given all this, I usually cut a size to suit my legs and hips and then I add a bit of fabric to the crotch curve while shortening the crotch length substantially. Depending on whether my fabric has some stretch to it, I may also add an inch to the waistband measurement. I do this on a case-by-case basis.
For Kwik Sew 3115 (yoga pants), for which I cut a medium, I made my usual alterations to the crotch curve and length. Interestingly, I didn't need to shorten the pants legs even 1 cm. Thing is, during the version that I made last weekend, I shortened things about half an inch too much, so the crotch of the pants is going up my butt slightly, and there's also some camel-toe action happening. It's not extreme, but for my next go-round, I've added back 0.5 inch of length to the crotch and shortened the hem by the same half an inch.
Even with a firm knit, these pants are not tight in my upper leg or hip, in the way some yoga pants are deliberately designed to fit. They skim. But since I'm sleeping in them - as well as doing everything else chez moi - I don't feel the urge to narrow the hip or leg.
Point is, I've opted for a medium, rather than a small, to better suit my waist measurement and to give me a flow-y fit.
One other thing I've done - to accommodate that lower ab bump (I will disclose that my whole abdominal area has been a ball of mushness for the last while - I sense it's hormonal and I am working to strengthen my core) is to cut a slightly longer piece of elastic (1 inch longer than the pattern calls for) and - this is key - I use 2"-wide elastic, not the 3/8" width that the Kwik Sew pattern calls for.
Why? Well, I don't know about you, but I think it's a bit weird and wussy to have a skinny piece of elastic sewn into the top of a 2.25 inch wide waistband. It also does nothing to support one's waist and below. The thicker elastic gives a nice, smooth waistline.
It's harder to insert, for sure.
What I do (and keep in mind that all elastic has different stretch properties which depend partly on width and partly on the material used to produce the elastic) is to sew a zig zag stitch (as the pattern instructs) not through the middle of the elastic but on the top and on the bottom of the elastic width. The trick is to affix the initial row of zig zag without rippling the fabric or accidentally catching it in a bump. It's trickier than it seems. Sewing the second layer of stitching is much easier, as you already have a fixed point. But you still have to be careful not to catch the fabric.
At any rate, this is how I've adjusted my yoga pants, for my unique shape. How do you alter stretch pants to do the same? Do you use a wider elastic to support the waist? Are you still working on that elusive sloper? What's the hardest part of getting pants to fit for you? Let's talk.