Asw... , doing the drains the way you say is great. As for the returns, not so great If you have the same spa that we install on our liner pools, remove the venturi fittings (6) and the fill fitting by the steps. If you need some info on how to do this, feel free to ask (we have ~40 of these that we winterize). While blowing out the lines, use a #11 expandable rubber plug to seal them, first remove the air inlet valve, this will allow you to get as much of the water out of the lines as you can. When you plug the last inlet, water will 'gush' out of the air port, keep blowing - put your palm over the air port and hold it there for a few seconds, then release it you should see a plume of water errupt, keep doing this until only 'mist' is comming out. At this point you can add antifreeze to the air port to prevent any water still in the line from bursting any pipes. You should then refill the spa (we've had a number of them 'float' in the spring thaw and water in them gives them more weight). Sorry not to go into more detail now, but detail = not posting this til after midnight
If you need more on what I've suggested, ask and I'll get to it tommorow.