Need help with longies instructions

Please Bare with me, this is my first attempt…lol
I am using this pattern

What I do not get is this.

Cast on needed stitches (5 for gauge x waist size ~ example medium waist of 17x5=85 stitches. Round to even number = 88) 16" size 5 needles. Join circulars making sure your work is not twisted and knit 2 purl 2 repeatedly for 12-16 rows, based on your preference for the amount of ribbing. Move stitches to 16" size 8 needles.

I have my stitches cast on to my 5 needle, but how do I Join circulars? Does joining circulars mean both, If so what size?
I am confused…lol This is the only part of the pattern that confuses me.

you will be joining to knit in the round rather than knitting flat. (i.e knitting in a tube not a flat piece).

once the stitches are on your needle make sure the working yarn (the part that is attched to the yarn ball) is on the right hand side. Then just start knitting as usual. This will join it into a complete circle that you just go round and around.

And when it says to make sure your work isn’t twisted, it means that you want to make sure all your stitches on the circular needle are straight and that none of them are “turned over” on the needle.

This article has a good picture of what it looks like when your stitches are twisted: