[Style Arc Barb Stretch Pant] Pants fitting: how do I g Sewing Machines Parts and Sales in Los Angeles

I just made a muslin of Style Arc Barb pants. The front fits pretty well. The back center waist is way too high, so I know how to cut that down.
The issue I’m having trouble with is the back thigh area, starting about 6 inches down from the top (excluding waistband). There is just too much fabric back there, it’s all baggy. I have this same issue with all store-bought pants, I think because I have thin thighs and a flat rear.
Do I alter just the back pant piece? Do I make it skinnier on just the outside of the leg or both the outside and the inside of the leg? I’m worried if I make an alteration incorrectly I’ll end up with twisted legs or something.
Thank you!

(PS – how do I add a photo?)