There are two yarns: K used for knit ribs (white) and P used for purl ribs (green). K yarn is held lower than P yarn.
Assuming your first stitch is in K:

Step 0 (not shown)
In case of in-the-round knitting:
Make a yarnover with K, purl the first stitch with P yarn and pass the yarnover over it. Tighten P yarn if needed.

In case of flat knitting:
Slip the first stitch.

Step 1
Make a yarnover with P, purl the next (P) stitch with K yarn and pass two P loops (stitch+yarnover) on the right needle over the third one (K). Be careful as the yarnover will want to unwind. Tighten K yarn if needed.

Step 2
Make a yarnover with K, purl the next (K) stitch with P yarn and pass two K loops (stitch+yarnover) on the right needle over the third one (P). Tighten P yarn if needed.

Repeat Steps 1-2 and fix the last bound-off stitch as usual.

Example: Autumn is Coming