Blocking is ironing your project so it looks finished and lies flat. You do this carefully, by first pinning it into the desired shape then using an iron set on Steam - and a wet dish towel [without the nap].
After pinning it, you wet the dish towel, wring it out and place it over whatever part of your item it covers. Then press it with the steaming steam iron, using gentle pressure and lift-press; lift-press type of technique.
When you've ironed the whole towel area, if you still need to iron another portion, you re-wet the towel, wring it out, place it over the unironed portion and iron. Repeat until all of your project is pressed.
Now leave your ironed treasure lie as still as possible overnight so it can dry and cool and generally settle into its new forever-shape.
Okay. Now on to your afghan.
Do this same technique. But I would take a CLEAN bedsheet and place it over some towels laid out over your rug. Then you can stick the pins into the towels/rug and have them hold the afghan in shape. If need be, you could fold it in half - but this would result in a crease where the fold is.
Well, hope this helps.
PS - do be sure not to walk on your new afghan while it's on the rug.
PPS -CONGRATS on finishing a whole afghan!!! What pattern did you use?