Use a pair of pajama pants that you already have and fold them in half. Iron if necessary to make sure you have the right shape.
Lay the fold of the pants on the fold of the fabric.Pull the crotch area out a bit so that it is pointy.
Cut 2 pieces (one for each leg) on the fold, using your old pair as a guide (you can trace the cutting line onto the fabric before cutting if you'd like).Note: Follow the CUTTING GUIDE in the notes section.
Fold them inside out and sew (straight or zig-zag stitch) along outer leg length edge (from the bottom, to where the crotch begins to curve). Trim close to seams.
Turn one pant leg right side out and stuff it inside the other pant leg. Right sides of the fabric will be facing each other. Make sure that the long seam that you just sewed on each leg matches the other.
Pin in the center to keep it in place.
Sew the crotch seam of the pajama pants using a ⅜" seam allowance. Begin at the waist, sew along, and end at the top again. Sew once more close to the seam line to reinforce, since this area will get a lot of wear.
Turn pajama pants right side out.
The length of the elastic should be the same size as your child's waist + 1".Fold down the waist ¼" inch and press it.Fold it another 1 ¼ inch and sew the casing all the way around with a ¼" seam, also leaving a 2" gap to insert the elastic into later.
Insert your elastic through the waistband using a safety pin to help guide it. Feed the elastic all the way through and out the other side of the opening.
Sew the ends of the elastic together. Stretch the waist, pulling the elastic back into the opening.Sew the gap in the waist closed.
Fold the bottom hem up and press, then fold it up one more time and press. (This one is about ½"). Sew around on both legs. Flannel is so easy to work with. You don't need pins since it pretty much just sticks together.
Video
Notes
Cutting Guide
Leave at least ½" for seam allowances on the sides.
Add 3" on the top for the elastic casing.
Add 1" for the bottom hem (can make it larger or smaller if you want).
Be generous when cutting around the crotch area (where the back comes to a point) so that there is plenty of room to bend over and move around without stressing the seam. I usually go at least an inch away instead of ½" for the seam allowance in the crotch area.
Note: Since you are using an existing pair of pajamas, the elastic waist is already scrunched. You can also add an inch or so to the cutting area if you'd like for a bit of extra room. If the existing pajama pants are shorter than you'd like, add length when cutting out the pieces. If they are too narrow, add a larger seam allowance when cutting out the pants.