Disclaimer: I am by far no expert on sewing and I'm sure there are a ton of different ways to alter jeans. This is just the easiest and most convenient for me.
These 7 for all mankind jeans were a steal at $3.99. I bought them knowing that I would have to alter them. The legs needed to be taken in and the length needed to be shortened.
First step is to turn the jeans inside out and lay them on the floor/table/whatever surface you like. Find a pair of your jeans that you love the fit of and lay them on top of the jeans that you are going to alter. Note: Line up the insides (starting at the crotch) of the jeans perfectly so you will just need to alter the outside.
Put your pins in the jeans that need to be altered along the edge of the jeans that fit you well. This will be your guideline when you start your sewing. I prefer to start sewing from the top of the jeans and work my way down to the bottom (but either way you feel more comfortable should work). Zig-zag stitch was my preference and turned out feeling very secure.
After you sew each leg, turn the jeans right side out and try them on to make sure they fit correctly. Once the jeans fit the way I wanted them too, I got to working on the length. This is by far the easiest part. I literally just tucked in approx. 3 inches of the material up into the jeans and then sewed all the way around each leg.