How to Join the Round on a Circular Needle
When you knit in the round on a circular needle, you first cast on, join the round, and then begin knitting. Joining simply means that when you work the first stitch, you bring the first and last cast-on stitches together, joining the circle of stitches:
Place a marker on the RH needle before making the first stitch if you want to keep track of the beginning of the round.
Many in-the-round patterns tell you to place a marker to indicate the beginning of a round. When you’re doing color work or any sort of repeating pattern, knowing where one round ends and another begins is vital.
Insert the tip of the RH needle into the first stitch on the LH needle (the first cast-on stitch).
Knit or purl as usual.
If you have to place markers later in the knitting process (common with pieces that require shaping), do something to differentiate your beginning marker from the others: Make it a different color than the other markers you use, or attach a piece of yarn or a safety pin to it.