These would also work as "boy shorts" for a girl... with girlier fabric... I just wanted to use D's old baby clothes...
*a knit cotton t-shirt that is too small (I used 18mo, 24mo, and 2t shirts for my size 6 3yo boy)
*scrap of cotton knit with Lycra/Spandex (same thing... the easiest way for me to tell if there's lycra/spandex in a cotton jersey is by stretching it... plain cotton=not shiny, lycra/spandex= shiny when stretched.)
*scissors, sewing machine, thread etc.
Start with the T-shirt... cut it apart just below the armpits, straight across.
The bottom of the shirt will be your boxers, the top will be cut apart even more to make the crotch gusset
(this tute is illustrated with 2 different pairs of boxers... I made the striped one first, then made the red one to get extra pictures of things I missed the first time)
Using the leftover top of the shirt, cut out a crotch piece. (if your kid is chubby, make it wide, if your kid is skinny, make it narrow. I'd aim for about 3" by 7", (but it's better to cut a piece that's too big, as big as your shirt will allow, and then cut down later, rather than cut a little piece, and have to piece together the crotch)
You can see the outline on the red shirt of the crotch piece... cutting that gave me 2 matching pieces (one from the front, one from the back). The striped shirt had the front collar placket, so I could only get one crotch piece from it... I supplemented with a part of a sleeve from another cotton shirt...)
*Try to make sure the crotch pieces are 100% cotton (avoid baby yeast infections!!!)
so now you have the bottom of a shirt, and 2 matching rectangles...
Cut a strip of the cotton with lycra/spandex that's 2-3 inches long (I like thicker waistbands for my son, they're easier on him) that's 3 or so inches smaller than the actual waist measurement (rough guideline: I'd cut 15-16" for a 3-5y/o, 16-18 for a 6-8 y/o)
sew the strip into a band, right sides together
This will be your waistband, and the lycra/spandex will provide enough elasticity that you don't need to actually put itchy elastic in the underpants! Fold the waist band in 1/2, wrong side together, so the seam is in the inside, and you have a ring.
Pin it to the cut/unfinished side of the shirt bottom, matching the waist seam with one of the shirt side seams, matching the 1/2 and 1/4 points of both. The shirt bottom should be bigger than the waistband, and you will have to stretch the shirt to match the waist band as you sew.
Ok, now it should look like this: (you could stop here and have a toddler tube top, I suppose)
Now fold it in 1/2 like this:
Grab your scissors and cut out a little crotch "J"
I just freehanded it, but if you've never made pants before, cut out a little J shape that's about 2" high. (maybe start with 1.5" and cut off more if needed...
Here's what the red pair looked like when I opened them up:
They look underpants-esque already, don't they?
Now go grab those crotch gusset rectangles, and serge/zigzag them, right sides together, on the NARROW ends, like this:
Turn the gusset right side out, and zigzag a little topstitch on the narrow ends like this: (don't worry about making the beginning/ends look good, they'll be covered in the next step)
Pin one long side of your gusset to the curve on one side of your boxers, right sides together. I find it easier to pin on the gusset side. First put the edges together and pin each corner, then the middle, then the 1/4 points, like so:
Flip them around and sew the other side, again, right sides together, pinning on the gusset side.
Clip threads, iron (if you care, I didn't bother, obvi).
Flip them right side out, and admire.
Wasn't that easy?!?!?!
Here's the set I made tonight... all have the same waistband fabric, therefore, obviously, they're a "set."