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

[Munsch]

SKIP the gussets according to the pattern. After the ribbing, knit four rounds.
Rnd 5: knit 3, pm, m1, k2, m1, pm, finish the rnd.
rnd 6 and all even rnds, knit.
rnd 7: k3, sl marker, m1, k4, m1, sl marker, finish rnd
rnd 9: k3, slm, m1, k6, m1, slm, finish rnd.

Continue until you have 10 stitches (for child's mitten) between markers. Knit one rnd.
Next rnd: knit 3, slip next 10 onto a stitch holder, increase with backwards loop for 2 stitches, continue.

When the mitten is done, put the 10 thumb stitches on needles, pick up 12 more. Continue in pattern as best as possible.