Gauge is determined with the US 7 circular needle, the US 9 is for binding off only.

These are knit 2-at-a-time, on one long circular needle (you can use your favorite method, dpns, or two circulars if you prefer). This pattern assumes you are familiar with casting on two open tubes

For further help check out this video I found: https://youtu.be/gu_4Kl6H984

Additionally, there are two books with different techniques, these are:

Circular Knitting Workshop by Margaret Radcliffe, or
2-at-a-time Socks by Melissa Morgan-Oakes.

I use a modified version of Melissa’s cast on, but please use the one you’re most comfortable with.