I just did this from how I thought it should be done. I can tell you here what I did though,
First I went to a thrift store and bought a big wool sweater with a nice easy to attach to knit.
Since I was making these two for our coming baby I didn't have an exact size I needed, so to maximize the usage of the sweater I measured across the front of for the main torso body shirt width (this will be your hip measurement) then I looked up corresponding average rise and inseam measurements for wool longies. (these are wool longies made for using over cloth diapers, so the measurements are different then if you disposable diaper).
Then I cut across the bottom at a little below the total rise + inseam measurement (I actually measured out about what I would need for the rise of a soaker for tris from the top of the sweater, which was a loose turtleneck, so that I would have enough material to make tris a soaker) my cut was a little angled but that's ok because you can make up the missing length when you crochet or knit on the new wool yarn to make th waist band.
I also drew in how everything else is divided up about-
Then I then measured to the middle of the fabric from each side seam and cut the fabric into two equal halfs each with a seam in the middle of it.
This got me two pieces that are both as wide as the sweater torso width with the side seam in the middle (i want to mention that on this knit sweater the seam can only be seen from the inside)
Then I used thread matching the colors of the knit to sew the cut edges together. you want to place the material so that the outside is folder against itself and the inside is facing outwards and sew like that, so the seam will be on the inside of the finished product. the nice thing with knit is if you do this carefully the seam is pretty hidden.
So you end up with a piece that is a tube lengthwise up and down, like a skirt. The bottom of this piece is bound off from the original sweater being bound off, the top is cut material and you have seams on either side that divide the piece equally into front and back.
Keeping the piece inside out I then measured out the length of the inseam I need plus about a half inch or a little less half way in them middle of the two side seams. I also cut over from the middle cut at the top just maybe a fourth of an inch. This allows for room for the seams to allow the crotch area to match up with the inside seams.
Still keeping the piece inside out I then sewed up the cut edges with matching thread to make a crotch and legs.
then you just knit or crochet on the waist band. make sure you attach the needle or hook to solid loops. You may have to sew some of the ends of existing loops to anchor a row if it got cut. pull out the small piece in between each loop that have both ends severed to make it easier to see what you are doing. Knit or crochet the waist band to be double the height you need it to be to finish off your rise.
then crochet or knit out a string to use as the draw string and position that so it goes all the way around the inside of the waist band and the strings come out at the middle and top of where the actual top of the waist band will be.
then fold the waist band down in half and finish by attaching it to the bottom inside stitches of the waist band so you have the string fully enclosed in the waist band. Remember to keep you stitches loose so you have good stretch.
Add anything you want to them like my snowman and snowflake, or just leave them as is.
You're Done! WAY TO GO!
Repeat for to the other pair of medium longies.
Here's a couple more I did, the soaker was from the same sweater as the longies above, the longies from a different sweater.