The thumb placement box is shown on the mitten hand and within the thumb chart for reference. The markers are placed outside the box. All thumb gusset increases occur between the markers.

While you are working the thumb gusset, on the rows where the number of stitches is not a multiple of 6, there will be a discontinuity where the edges of the thumb and the rest of the hand meet (at the markers). This has thrown a few people, so just know to expect this. If you follow the thumb chart, and use the markers all will be fine.