I have always wanted pants like these. I draw the pattern from a pair of my friend's pyjamas trousers. They are just perfect for relaxing! They where very cheap, because I used an old duvet cover.
You will need two peaces of fabric looking like the once on the picture(one for the front, one for the back) I draw my pattern from another pair of pants, but I will try to describe how to measure it. This will probably seem confusing, but keep your head cool!
Top: half the measure of your hips(plus 5 cm to make sure it will fit over your butt)
Length: Measure your legs from your waist (not hip) to the ground (add 5 cm again to make sure they will get baggy)
Bottom: Measure your ankles and (again add 5 cm.)
You don't have to take care of sting allowance because of all the added cm.
Draw the center of the pattern using the length. Center the top measure and the ankle measure.
Measure 30 cm down from the waist and go 5 left or right. Draw a soft curve using the marks.
Combine the 5 cm mark with the ankle.
(I am a size M and my measures were top: 2x50 cm, ankles: 30 cm, Length: 120 cm)
First I would zig zag all edges.
Then: Fold each leg piece in half, right side together, pin from ankles to crotch and sew (do not sew all the way up to waist!)
Make a "track" for elastic bands in the ankles. Feed the elastic bands into the track
Now put the unsewn edges together. (from the crotch to the waist) Pin and sew. I couldn't get a good picture of this, so I hope you know what I mean.
Don't sew all the way up in the front. leave 3 cm. Bend it around for making a "track". Feed shoelace for tightening in the waist.
There you go, ready to wear. These where the second ones I made for my friend. I hope you understand the How-To. If not, please tell me!