Here are the supplies I used:
I used 1/2 inch wide elastic, a t-shirt with holes in the armpits, some tailor's chalk, scissors, and my sewing machine with my zig zag foot. And a pregnant belly.
I tried on the pants and made a careful cut down the front until I was comfortable. When I do this again, I'll add about 1 inch to that to better account for the seam allowance and a little extra room.
Using the tailor's chalk, I marked approximately where I wanted the belly cutout to sit up to the sides of the scrub pants. I made a few adjustments until I was happy.
I decided that, as convenient as I thought this elastic was when I bought these pants, it need to go. So I whacked off the casing all along the back.
Ta da. Again, I'd just dip it down in the front a bit more next time. Also, these pants have pockets and I cut off whatever bit of the top of the pocket that I needed to to get to this point, but I kept the pockets. I sewed up the top later.
You can see below why I chose this shirt to donate to the cause. It has been well loved. I also chose this shirt, because in my pregnant state, it was nice and snug around my belly with a tad of that knit fabric flex.
I cut straight across the shirt from armpit to armpit. I'll save the top for my ridiculous plans for a gazillion more t-shirt quilts.
I went ahead and put the pants back on, this time with the shirt bottom to make sure it was a good snug fit at the t-shirt and comfortable enough on the pants. You can see that it's a little gaping at the top even though I know I'll be cutting it off just above my belly (I was going for over the belly on these, but I find that the t-shirt folds down nicely for an under-the-belly look, too)
While I had the pants on, I went ahead and pinned the shirt to the pants at a few spots on the bottom, and put a few in to mark where I wanted the elastic to sit on my belly at the top. I also decided I might have to add a dart to make it a little less gaping at the top on the front.
Back up in the sewing room, I took the shirt and folded it down over the pants so they were right sides together and repinned where I had marked before.
See my pins?
I went ahead and flipped the pants inside out enough that I would be able to sew with the pants on top and follow my nice cut. Here you can see the layers of the pocket I left. See? They're going to get all closed up just like before. No problem.
I sewed about a 1/4 inch seam all the way around.
Then, with my trusty zig-zag foot, I zigged and zagged the edges of my seam for a bit of support. If I had a serger, I would have serged away to do this all in one step.
Here's a really blurry iPhone picture of my zig zag. Sorry.
Now, you can flip the shirt up, and it's starting to come to life!
Now, originally, I saw a tutorial somewhere where they planned everything right and just sliced open the casing in the hem of the t-shirt, fed their elastic through, and they were done. My original hem was not wide enough for the elastic I had in my stash, so I made a new one.
I cut off the excess based on my pin markings while I had the shirt on, adding about 2.5 - 3x the width of my elastic to make the casing.
It's hard to see here, and I didn't take a pic to demonstrate, but I put the pants back on, folded over the excess that was gaping and marked where I needed to sew a seam to make a dart (is that even the right term?)
Here, I had cut off the excess leaving a 1/4 inch seam, which I also zig-zagged.
I folded over the top of the shirt about 3/4 in plus a little (my elastic was 1/2 in), then folded over again to make a casing.
This is my casing, and a barely identifiable seam down the middle of my belly. I then measured around my belly with the elastic making sure it was going to be snug with room to overlap the ends to sew them together.
Of course, I left an opening in my casing to feed my elastic through.
Post-elastic-feeding. Then I sewed the ends of the elastic together and sewed up the hole.
And here are the pants after an eventful day of dentistry.
They were sooooo much more comfortable. And since it's 130 degrees F here in Texas, I found that folding down the t-shirt into a band works just as great and is much cooler : ) I'm glad it got bad enough that I decided to do this. I made my first dart (if that's in fact what it's called), my first casing, and refashioned a piece of my actual clothing : )