Personally I prefer not to wear cute looking clothes to work, but these didn't even fit. I figure this is a pretty common issue for the ladies, but couldn't find any simple instructions online. (Plus work clothes in women's cuts usually cost way more-- LAAAAAME!) So here you go, buddies!
(This method can be used to take in and adjust just about all clothes. I've done it to take in tshirts, shorts, pants, coats, pants, to make dresses more shapely, to make pants for much taller people fit me well. Don't be scared!)
Step 1: Find your size
First, put the pants on inside-out. This way you can see all the seams and get a good idea of how your pants were assembled. The best way to alter clothes is from the seams that already exist. Just pinch the pants into place and safety pin them when you get the shape you want. Be sure they're still comfortable when you sit, bend down, hop a fence, etc. Keep the pins in a line so it's easy to keep track once you're sewing.
The pants I'm altering fit at the waist and butt, but the legs were way too loose and went down too long. So I'd need to first take in the legs and them hem them. When greatly altering jeans, I've had the easiest time and most success from taking in the outside seam. However these ones are triple stitched flat, and since the top part fit well enough I didn't want to try to blend one type of sewing with another and end up with lumps where the stitches meet. Pinching them from the inside pulled the front in a weird direction. So i chose to just create a new seam and take them in from the back of the leg. This isn't how pants are made, and probably isn't how I'd alter fancy pants, but it turned out to be an effective way to fix these.
-Once pinned, quickly machine sew along the line with a straight stitch at the longest length (technical term: "basting"). Or do it quickly by hand.
-Take out the pins and try them on with the right side out to make sure they fit how you want. If they're not good, just turn them inside out and pin/baste them again until you're pleased.
-Once you have the fit you like, sew them for real. I used a double thick stitch for super reinforcement. A straight stich with a zigzag stitch next to it, or two parallel straight stitches with a shorter stitch length would also work well. If sewing by hand use very sturdy thread. Dental floss is an excellent choice because it will never break. You don't even need to sew it that neatly for dental loss to do the job well.