This took me less than 20 minutes from start to finish. Obviously I've got experience doing this so it doesn't take me much time at all, but it's really not hard. I know you can do it, I've got faith in you!
I started out with a pair of size 20 Gap skinny jeans. Also starring: my belly.
The problem with clothing sizes is that most manufacturers size the patterns up all the way around, rather than in certain areas. This is why hems and sleeves get longer when you go up a size, and why bigger jeans sometimes have larger legs than necessary.
Now these jeans were more of a straight leg on me, and they looked all right, but I wanted a pair of severely skinny jeans. The kind that cut off circulation in your legs. They won't be incredibly flattering, but I don't really care.
I immediately encountered a problem. Because these are distressed jeans, there's a hole right about where I want my seam to go, so they're going to be bigger in the thighs than I want. So it goes.
Put your jeans on inside-out.
The first thing to do is get some kind of marking device (I use Sharpies, because I like Sharpies) and pinch the seam as tight as you want it to go at the highest spot on your thigh. From there, move down to the knee and mark there. Mark again at your calf and then finally at the hem. I start with the left leg, because I'm right handed, obviously you can do what works best for you.
Take the jeans off and lay them out somewhere you've got a lot of room. Then play connect the marks.
Next, I line the legs up on top of each other exactly so the mark is facing the other (unmarked) leg.
Then fold the top leg back at the highest point of your mark, exactly on the mark, and mark the leg below it. Do this every six inches or so until you get to the hem.
*NOTE: You can mark each leg separately like you marked the first one, but I've found this is less accurate and it takes longer.
Play connect the marks again on the other leg.
Now it's time to sew. Doesn't matter which leg you start with, I usually start with the left. Dunno why, I'm just odd that way. Follow that mark with a seam all the way down the leg.
Now, because of the way you marked them, you'll be starting at the hem on the other leg. Run your seam down the mark.
Try them on!
Because of the excess fabric inside the leg, they'll feel tighter than you were expecting. You can also inspect them for strange bumps and wrinkles. I've got one on the left thigh. All that needed was for me to run a short straighter seam to get rid of it.
You can also try them on inside-out to check the fit. You'll look ridiculous though.
Cut the excess off with pinking shears. You can use regular scissors, but you'll get a lot of loose threads that will need to be snipped off after you wash them. Cut about a half inch away from your seam.
Put them on. Be proud of yourself!
Maybe later, think about ironing the new seam flat to make them look proper.