Knit to your increase point, knit the stitch with both colors, and you've worked the increase! For the gusset I increased after the first marker and before the second marker every round. I really like that it doesn't draw pull the work vertically like a lifted increase.
One downside: both colors of yarn are slightly visible at the base of each stitch, especially before blocking. Your increase also cannot be the same color as the stitches on both sides.
|Insert needle to knit
|Wrap both colors of yarn around the needle
|Pull both strands through and one stitch has become two
|After working 3 inc's on each side
|Other side of gusset
|The finished gusset