-
For garter stitch: Alternate a knit round with a purl round.
-
For stockinette stitch: Knit all rounds.
-
For rib stitches: In round 1, alternate knit and purl stitches in whatever configuration you choose (1 x 1, 2 x 2, and so on). In subsequent rounds, knit over the knit stitches and purl over the purl stitches.
The trick is knowing how the stitch is created in flat knitting and then remembering the principle. For example, in flat-knitting seed stitch, you knit in the purl stitches and purl in the knit stitches. Well, you do the same in circular knitting.
Rounds (rnds) are what you work in circular knitting. Rows are what you work in flat (back-and-forth) knitting.