Fitting trouser muslin

[DrLang]

I stretched the left side seam and the back fork a bit. And I let out about 3/4" at the crotch. So maybe a 1" total gain in crotch width. It's definitely an improvement, but I'm about ready to throw the towel in on this pair. I have run out of inlay and I can't let out the hips anymore due to having slashed the inlay below the pocket opening. Not to mention that this light weight cloth is so unforgiving with every fit issue.

So for the next ones, just to make a reminder for myself, I am going to extend the back fork and inch, add 1/2" to each hip divided between the sides and center of front and back, and I am also going to try and add some extra ease at the top of the backs and either shrink down the waist or take more into the darts. Make the fly at least an inch shorter. I definitely took it too far here. And don't slash the side seam allowance until after I'm finished with fitting.

[peterle]

A bit of inlay would be fine, yes.
In the profile pic you can see that the pants front curves inwards and is pulled to the body between the waistband the thigh, thus causing gaping pockets. You need a bit more width there so the pants can fall a bit more loosely in this area. You can also see the horizontal stress folds in this area caused by lacking width. Instead of adding width to both sides I would prefer to create more width by slashing and pivoting the pattern.
The undersides have some folds below the hip line pointing slightly inwards. This tells me, the top undersides  are cut a bit too slanted. Straightening the top a bit will probably clean this area. When you have a enough inlay at the back seam, you can yet alter it. (Releasing about 2x 1,5cm at the back seam an taking 1,5cm in at the sides seams)

[Hendrick]

A bit of inlay would be fine, yes.
In the profile pic you can see that the pants front curves inwards and is pulled to the body between the waistband the thigh, thus causing gaping pockets. You need a bit more width there so the pants can fall a bit more loosely in this area. You can also see the horizontal stress folds in this area caused by lacking width. Instead of adding width to both sides I would prefer to create more width by slashing and pivoting the pattern.
The undersides have some folds below the hip line pointing slightly inwards. This tells me, the top undersides  are cut a bit too slanted. Straightening the top a bit will probably clean this area. When you have a enough inlay at the back seam, you can yet alter it. (Releasing about 2x 1,5cm at the back seam an taking 1,5cm in at the sides seams)

Top, Peterle! With all that in mind, next time it would be wise to toile the trouser before wasting cuttable material. When you fit trousers like these, in a "nervous" like we say, tropical wool you are struggling with a draft and a critical material at once... Use something sturdy, like a light canvas (I used to use the stuff that is used inside couches and other furniture, I believe Anglosaxons call it scrimming or something like that). Mark the draftlines on your toile; it will be easier to note defects... Adding volume (well distributed) like Peterle said will also relax the backparts. I mostly fitted ladies'trousers but there is one last note; beware of taking out too much from the back darts; this will cause your trouser to crawl upward again. And worse it will pull the upper welts of your backpockets open. This will cause your backpocket welts to look like they hang by the buttons. Generally, and I think that is where Peterle goes, volume covers prominence. But again, in mens' trousers I am only a tourist.

Cheerio, Hendrick

[DrLang]

Instead of adding width to both sides I would prefer to create more width by slashing and pivoting the pattern.

I am trying to picture this in my head and I'm not really getting it. Where would you slash and pivot to add width?

[peterle]

Something like this.
The hinge points are marked red.
This will create a bit more space for your upper thighs (the pocket fabric pokes through) and smoothes in to your original waist width.
Probably you could shift the slash line away from the center line towards the side seam. Depends on, where the pants cling most to the thigh. Hard to tell from just a profile pic.