you are making increases for the thumb
you have a rectangle --in the middle (well not the perfect middle, but close to middle) you are going to increase.. a V
|vuvuvuvuvuvu|--straight edge ribbing will become:
|vvvvvv\vv/vvvvvv|--the [COLOR="red"][/COLOR] and [COLOR="red"]/[/COLOR] are the markers
(and the markers are Idiot points--(hey idiot--do something here!)--i use them all the time, cause i am an idiot too!)
so ever right side row, you come to a marker (hey idiot, do something) and you increase(after the marker)
then you knit some more and come to another marker, and you increase again (before the marker)
on the purl (wrong side) you do nothing!
after a few row, when you are mid row, fold the knitting in half..
and you'lll have
a wedge (in the middle of the row)!
this is for the thumb..
(you hand is wider at the base of the thumb than it is at the wrist..this wedge (increases) makes the mitten bigger..
(and after you've finished the wedge, you'll set it (the stitches between the markers) aside--you'll put them on a stitch holder.
|vvvvvvvvvv/[COLOR="Red"]vvvvvv[/COLOR] those red stitches will be put onto a stitch holder..
and you'll cast on 2 or 4 (or some number) to fill the gap between the front and back.
(remember those first 2 stitches between the markers, back in row 1 of increases? they are now on the stitch holder!--you will have to cast on some stitches to make up for them!) and a bit more.. (because your hand is wider above thumb than it is at the wrist.. even with out the thumb!
If you put the thumb stitches on thread (and not on a stiff stitch holder,) you can knit to half way point, fold knitting and poke your thumb out of the thumb hole (and it will make sense, even with the side (pinky finger side) seam open.
It is hard to envision.. I hope i have helped!