To begin with, Take the measurements seat/4+ 3 inch.
Fold the cloth such that you obtain 4 layers according to the seat measurements.
Mark the length ( see the draft) . Mark the shape of legging at the open side.
Mark 2.5" down from top of the edge for waist belt.( my waist belt is 1" i have taken double of the belt size + 1/2" for folding inside) from there measure down the full length and mark(G-H). and again mark 1" down for folding.
Mark 3/4" from the open side as A-C in fig. 1
Mark the crotch (seat/3) B-D then draw line to connect C.
As fig E-F shows the knee circumference and G-H ankle circumference + 1". draw the slightly curved line from crotch to ankle as in fig and cut along the line.
Fig 2. I attached the gore pieces at the crotch point according to my comfort. if you wish to attach cut the gore in 2 pieces,otherwise no need.
After cutting the fabric you obtain this shape.
sew the edges of the leg piece together with 1/2" seam allowances from one leg piece bottom edge to another leg piece bottom edge.
Again fold by casing width which you desired as in fig 12 and stitch around and leave 2" for insert the elastic as in fig 13.
Fig 14 ,15 shows the the elastic inserting the casing, pin the elastic one ending and insert the pin and pushing it through the casing
When you push the pin inside the casing you will see that it makes more ruffle,push the pin until the elastic comes outside , stitch the both elastic edges together and stitch the casing open.
I have stitched one line around over the elastic casing.