help with briggs little thrummed mittens

[ctmax]

I am knitting the thrummed mittens and do not understand the pattern. I am knitting the 4-7 years size and it tells you to cast on 28 stitches and then after you have to start casting on 2 at a time for the gusset. 30 stitches doesn't work with the thrum pattern. I'm not sure how to keep the thrum pattern going after you start adding stitches for the gusset? The pattern is briggs little family thrummed mittens #105