'ending with RS facing' means the last row you do before the next set of instructions should be a right side row (or what will be the outside of your legwarmer). In a simple rib pattern like this, it may not matter which is RS and which is WS (wrong side).
'cast off in ribbing' means that while you're casting off, knit the knits and purl the purls. Knit 1, knit the next, pass the 1st over the 2nd, purl the next, pass the previous over it, purl the next, pass the previous over it, knit the next, pass the previous over it.... etc.
Mommy to two wonderful girls
"I don't have to conform to the vagaries of time and space, I'm a loony!" -Campbell Bain (David Tennant) 'Takin' Over the Asylum'
OTN: Cabled Sweater
, arm warmers, kids' clothes