Without seeing the pattern, I'm just guessing that there might be a band which comes up from the front and goes around the hood. I'm also guessing that there might be a band on the other side as well - so you could end up with two sets of stitches on stitch holders ???

Purl the 12 stitches, using the same knitting needle you have been using, then transfer the 12 onto a stitchholder.

The yarn will be at the base of the 12th stitch - just pick it up and continue to use it for working on the remaining stitches.

The instructions will tell you to work on the remaining stitches until that bit is finished and you will probably have to cast/finish off.

At some point, the instructions will tell you to join the yarn at one end (and will probably specify which end) of the 12 stitches - at this time, you will slip the stitches from the stitch holder back onto your knitting needle, slip the yarn through the loop at the base of the first stitch, tie a knot, and off you go again.

Stitch holder - a large safety/nappy pin or whatever you call them in the US would work - or use a short double ended needle and wind large elastic bands on each end to stop stitches from slipping off. You can even cut down a bamboo skewer. Whatever you use, have it just large enough for the 12 stitches - otherwise they can really get in the way.