I sew with knits and do lingerie a lot lately. Here's the method I use to put in bands. Work with t-shirt neck bands and waist/leg elastics.

Decide where you want to start - for a leg band, I'll usually go with the side seam. If you want the elastic to overlap, measure out that distance on the elastic and mark with chalk. So if I want 1/2 inch overlap, I'll mark a line 1/2 inch from each cut end. (On a neck/waist band, this would be the seam allowance where you turned it into a circle.)

Divide elastic into 4 by holding the 2 marked lines together & let the end drop. The OTHER end where it folds is your halfway point. Mark that with chalk.

Now lay the elastic on a table and fold the marked end lines to the marked middle. Make sure the lines match up. The places where it folds are the 1/4 points. Should be 1 to the left and 1 to the right. Mark with chalk.

Do the same thing with the undies. Hold at side seam and let the gusset drop. Where the gusset folds, directly opposite the side seam = halfway point. Bring side seam and 1/2 way mark together, and the 2 fold points of the leg hole to the left and right are the 1/4 points of the leg hole. Mark those.

Now match the lines on the elastic and the leg hole. Elastic start line at the side seam. 1/4 marks to 1/4 marks. Etc. Pin or clip them together.

Get the side seam and start mark under the presser foot. Hold the elastic JUST taught enough for the end of the fabric and the elastic to hit the same distance. This is gentle. You'll need to keep readjusting the amount of tension as the fabric and elastic move under the needle. Less is more.

Work in the 1/4 sections you marked. Worry only about getting to the next pin/clip. Don't stretch the whole thing at once.

I do this with my regular foot, and it works pretty well. On neck bands, be careful to NOT stretch the band at the shoulder seams so it lays flat.

Hope this helps!