You mean... RPing as an act itself? Well... I guess I could try my hand.
RPing, Role Playing, is quite simply the art of playing a role in it's simplest form. This is why there are things like roleplaying games. However, the form we do on the forum here is a bit different--a more specific kind.
I refer to it as text-based roleplaying. Text-based roleplaying is neat because of the intense freedoms it gives the roleplayer. You don't have to start as some preset character or follow some linear story if yuo don't want to. (Or hey, you can do that too!)
You can basically do absolutely anything, though for obvious reasons we have some rules restricting mature content. ;P
Now, the act of roleplaying is quite simple. It's much like writing a story, only you're only controlling certain characters (usually only yours and any extensions of your character such as your character's pokemon.)
Therefore, you could roleplay simply by writing the following:
Yellow walked down the street.
That's called emoting, where you describe what's going on via text, and it's the basis of all roleplay. Roleplay, in it's simplicity, is just a bunch of people doing that and interacting with other roleplayers doing that.
You can also use quote to indicate speech, much like a book, ie:
Yellow walked down the street, stopping to say hello to a particularly intriguing stranger. "Um... hi there!"
The rest is just making your post pleasant to read by inputting flowery, pleasant language and intriguing characters and stories!