Step 1 - Unstitch and detach entire waistband (including belt loops and label)
Step 2 - Unstitch leg seams, back seam, back yoke (you may not need to remove the yoke depending on how much you want to downsize the jeans - for bigger changes you'll need to remove it), and pockets (order does not matter)
Step 3 - Use a pair of jeans that fit you well to establish the crotch of the new jeans (front and back). Use pins or marking pens to mark how much fabric you would like to trim away. Remember to always keep in mind your seam allowance. These marketing can include seam allowance or you can include after, it is up to you.
Step 4 - Pin new side seams and try on the jeans making pinning corrections if needed. You should now have pinned a new crotch, new side seams, and no longer have a waistband or a yoke
Step 5 - Remove the stitching on the side seam where the pockets connect. You want to be able to remove the pockets.
Step 6 - Once the pockets are unstitched, you can cut the pocket out leaving a 1cm seam allowance. This will allow you to replace the pockets closer to the zipper. We do this so we can take the seams in at the side without interfering with the pockets.
Step 7 - Taking measurements from the pair of jeans you like again, use the crotch measurement to decide what type of rise you want for the jean (low, mid, high) and pin where you would like the jeans to hit. Keeping in mind that the waistband will also add height to the jeans. So this in the front and the back so you can see how much fabric you need to remove from the top of the jeans (always keeping in mind seam allowance)
Step 8 - Create markings (pin or pencil) where you would like the new pockets to lay and test placing the waistband to see how it looks
Step 9 - Re pin the yoke on the back of the jeans. Pin it down so that its ready to sew keeping in mind the amount you want to take in on the back seam which you will also be sewing in the next step
Step 10 - Re sew the yoke and the back seam of the jeans and if you are happy after trying them on, trim away excess fabric
Step 11 - With the zipper head down, trim down the front of the jeans as well, following your pinned markings from earlier, remembering seam allowance
Step 12 - Do a zigzag stitch at the top of the zipper to make sure the head doesn't slip off
Step 13 - Attach the pockets back on. With right sides together sew your pockets back on the pin markings that you established. I sewed half of the pocket, made sure i was happy with it's placement, trimmed away the excess fabric, and then finished sewing on the rest of the pocket. You can also trim the fabric away before you sew, this will make it a little easier to sew along the curve of the pocket
Step 14 - Because I don't have a serger I like to zigzag stitch all my seams, I do that now to the yoke and the back seam
Step 15 - Now I sew the side seams of the jeans, the legs. Pin first and try on to make sure before you sew (the front pockets, yoke, and crotch seams have all been sewn at this point)
Step 16 - I decided to take in the pockets (remove some of the depth because they were bulky), this step is optional
Step 17 - Trim away all excess fabric
Step 18 - Double top stitch the yoke and the back seam
Step 19 - Take in the waistband. I took mine in from three different spots to create a more rounded shape. This depends how much you need to take in the band. The more you take it in, the more of a curve you'll want to create (waistbands aren't straight, they're curved), and the more cuts you'll want to make to round out that curve so it's not all just from one spot (which would create more of a V shape than a U shape)
Step 20 - Reattach the back pockets onto your jeans (you can take them in a little if they are too big like I did)
Step 21 - Reattach your new waistband, top stitch the top of the waistband, and reattach whatever belt loops you decided to keep
Step 22 - Reattach the label and you're all done !!