So i started of with a pair of old pjs. and i basically just cut up all the seams.
Then I have this super duper cute fabric I got at in the US last year (my parents lived a year in Houston so I went and visited them before last summer).
So basically I just used my old pjs as a guide, but added quite a bit of fabric to the length. I actually find it easier to cut off the bottom so I know the bottom of the legs are straight.
Then I cut it out.
After that I sew it together. And sewed some more, and some more, and some more. I had to do a lot of altering. For the waist I just added an elastic band. In the end, it ended up like this: