How to give a scarf the scrunchie look?

I’m knitting a scarf for my sister, and this is my first one. I just changed colors and now I want where I changed the color to a new one to have a scrunchie - bunched together look for a few rows, how is that done, thanks

you COULD do short rows in progression
K10, turn, K(or purl)9 W & turn, K9 Turn, K(or purl)8 etc to about4, then reverse

OR
you could increase every 2 or3 sitches for a few rows then decrease at the same rate till you are back to your orriginal Number of stitches

just 2 ideas

ecb