The GARTER SLOUCH hat is gaugeless!
It can be worked in any yarn (with the exception of laceweight), using any needle.

It’s worked in the round, top down, in garter stitch using double pointed needles, and a 16” circular needle. 
If you’re a Magic Loop kind of person, you can use that method too, though I don’t give instructions for it here.

You launch into casting on the top of the hat with reckless abandon, increasing only on KNIT rounds, until there are enough stitches to take a gauge reading. Piece of proverbial cake!

To work this pattern you should know how to knit garter stitch in the round, and how to kinit using DPNs and a 16” circular needle.

You should also be familiar with one of these cast-ons:

Judy Becker’s cast on (my favorite) - as demonstrated by Cat Bordhi here: https://youtu.be/_Px2dxWHZ8U

Emily Ocker’s Circuluar Cast On
https://youtu.be/2gpnpQHAEfA

Turkish Cast On
https://youtu.be/ePELhLO2180

AND EZ’s Sewn Bind-Off:
https://youtu.be/GZjFzgb50Gs