I measured around the top of my pants
and drew a waistband with that length plus 2 times the seam allowance and 8-10cm wide (so the waistband in the end is 3-4cm wide)
next I pinned it to the pants
sewed, folded over, folded the seam allowance inside, pinned in place sewed all around close to the original seam
then I only had to sew the hem and button and button hole
and my pants are finished! they are just so comfy, I really like wearing them
so I made 2 more of them.
I hope this was helpful for you! Let me know if you have any questions.
keep sewing and smiling