A lot of people just say to use the new needle (the circ) and just start knitting the stitches off the old needles, but I usually transfer all the stitches before I start working. So, if you were going to use the 2 circ method, take your first circular needle and starting from the beginning of your round, slip stitches one at a time off the dpns and onto the circ until you have half of the stitches on the first circ. Then do the same with the second circ and the other half of the stitches. You can then proceed with decreasing in the round using the 2 circ method.
If you’re going to try magic loop, you’ll need one long circ. Personal opinion, 40" is about ideal for any project. Follow the same procedure as above until you’ve transferred half the stitches over, then pull more of the circ all the way through the stitches just transferred (but not so far that the stitches fall off the needle tip at the other end) and use the same end of the needle to transfer the other half. You’ll have half of your stitches on each of the needle tips and you’ll have a loop of cord between the two halves.
If you haven’t tried 2 circs or magic loop yet at all, you might feel better if you practiced on a swatch first before working on the actual hat - or if you’re brave, just go for it! Neither method is a complicated as it appears at first glance.