there are lots of ways to decrease in ribbing, but basicly they work out to
Keeping the ribbing intact, (as 2 X 2 or what ever pattern)
you select 3, 4 (some number) of ribs to 'eat up' the other ribs.
here is a hat
done this way
--this is a good style to learn, since it is also used to decreases in ribbing on sleeves, or shaping (waist, etc) on sweaters knit entirely in ribbing)
and
changing the ribbing pattern,
from 2 X 2 to 2 X 1, to 1X 1, to stocking knit, and done)
here is a hat done
this way
there are variation on the decrease process in both styles, but most decrease patterns fall into one or the other (there are some special patterns that only work with 1 X 1 ribbing)