Since my tummy is decidedly *not* flat, I've found that I really like the firmness and "hold-in power" of taut waistbands. I love how my jeans feel when I first put them on, but after an hour the waistband is stretched out and the tummy is flopping. On my last few pairs of pants (even non-jeans), I've been adding wide elastic to my waistbands and I love the result.
In Power Sewing Step-By-Step, Sandra Betzina shows a similar method. Mine differs, however, because I do not change the length of the original waistband. My waistband would still fit me without the elastic. The elastic is really just a very firm interfacing which acts to snug in both the waistband and my tummy. Sandra's method allows you to feast at the Thanksgiving table without undoing your pants. Mine probably doesn't. Keep that in mind in November.
To begin, I sew the waistband to the top of the pants as usual, and then press the seam allowances up toward the waistband, as shown in the photo below. This particular waistband will be folded over, but this method will also work with straight waistbands which have separate front and back pieces. The key is to cut (or finish) the waistband so that it is 2x the width of the elastic, plus seam allowances.
I used a fold-over waistband because I was also trying out Sandra Betzina's tip for cutting the waistband with the selvege as one of the long sides. If you're not using the selvege, overcast this inside edge of your waistband.
With the waistband attached to the top of the pants, it's now time to attach the elastic. I'm using 1-1/4" elastic. I have a 50 yd bolt of this stuff so you can guess that nearly all of my finished waistbands are 1-1/4" wide.
You can also see a bit of fusible interfacing in the photo below. I stop the elastic at the button/buttonhole areas because buttonholes are bad enough without contemplating making one through elastic! The fusible is to add body and stability for the buttonhole and button to be added later.
Pre-stretch the elastic 2-3 times and then cut it the length of your waistband minus buttonhole areas and then minus 3-5 additional inches depending on what's comfortable for you.
Slide the elastic behind the seam allowance as shown below. This is just so you know where I'm talking about. Once you know where the elastic sits and is stitched, you can skip to the next step.
Zigzag the elastic to the seam allowance as shown in the 2 photos below. Do not sew through the waistband, only through the seam allowance. Stretch the elastic as you sew. You may wish to quarter mark and pin it. I pin, but I just eyeball the quarters -- no marking.
To sew, flip the waistband down toward the pants so that both are on your left with the waistband on top. Then align the long edge of the elastic just inside the seam stitches and zigzag. This way the bulk of the pants are to the left of the needle, the elastic is to the right and on top so you can see where you're stitching in the seam allowance. Neatness doesn't count because no one (even you) will ever see this stitching when the waistband is finished.
This waistband has a center back seam and the resulting intersection of those seam allowances so I just skipped over that area, which allows the waistband to still fold down neatly in the next step.
Next, fold the waistband toward the inside *snugly* over the elastic and press. Although this is not hard, this is probably the trickiest part because you're going to be fighting the elastic wanting to gather up. Just fold over and press a few inches at a time. Again, the waistband police will not be ringing your doorbell.
Finish the ends of the waistband around the fly (or other) opening per your usual method, trim seam allowances and corners, turn, press and slipstitch closed. Then fold under the "seam allowance" of each end of the waistband at the top of the zipper and press. (I forgot to take a pic of this, but I will add one when I make my next pair of pants.) The goal is to clear the waistband long edge seam allowances away from the top of the zipper while also catching the top of the zipper inside.
From the right side, stitch in the ditch between the waistband and pants, catching the inside edge of the waistband. Use thread that very closely matches your pants, and start and stop stitching where you pressed the seam allowance under at the zipper in the previous step.
You can pin the waistband down in a few key areas or you can just stretch the elastic as you sew. I find it easy enough to stretch as I go.
Below is what the stitching looks like from the right side. It's more visible in this photo than in real life because the light of the flash is bouncing off the shine of the thread. Trust me, no one is going to be inspecting your waistband seam so even if your stitching *does* show a little bit, don't worry about it.
This is what the stitching in the ditch looks like from the wrong side. Notice that the seam allowance is *not* turned under. How fast and easy is that?? (Remember, it *is* turned under on the wrong side at the top of the zipper -- I just don't have a pic yet.)
This is the waistband off of my body. It looks like your regular bulky gathered waistband, doesn't it? Well, look at the next pic ...
This is that waistband on me. It's flat when I'm wearing it. My waistbands don't stretch out and become too loose because the elastic is keeping it snug, and my tummy gets a bit of restraint in the process. If only I could exhibit such restraint around the cookies and chocolate!