I use the magic loop to knit mittens. I learned this trick from Kelly's video on mitten making on Knit Picks about where to place the gusseted thumb. I come in 4 stitches from where the round begins and place the stitch marker , knit the next two stitches front and back to increase to 4 stitches, place marker. Then plain round . then increase by two stitches on each increase row separated by plain rows between increase rows. One the 2nd mitten, I locate the thumb 4 stitches in from what would be the side seam and repeat the increase rows. When increases have reached 14 stitches, place gusset stitches on waste yarn and knit 2-3 stitches to close that gap and continue in the round.
I hope this will help you.
Daylilies are the Lord's smiles, a new one every day.