This will show you how to fix that.
Step 1: Materials and Tools
A pair of jeans that fit your waist and butt
A sewing machine
A serger (optional but it makes things way easier)
Step 2: Step 1
This will feel funny if you have details on the pockets :D
Step 3: Step 2
Skinny fit: Pin them from where they jeans start to go away from your body to your ankles. Pin it close to your body.
Flared fit: Pin them from just above the knee to the widest part of your calf. If you have really skinny legs and your jeans start flaring away from your body higher that just above the knee start there.
If you are not flexible, you may want to get a helper for this part.
Pin your jeans along the previous seam. The seam that is generally on the outside of the leg is the one you want to pin.
Step 4: Step 3
Sew along where the pins are.
Sew along where the previous seam was before and gently and slowly curve out to reduce bumps when you put them on.
I generally baste first then try on the jeans and make adjustments and then sew on a normal stitch length.
Make sure to backstitch!