Here's my whole routine...
I normally have 10+ gallons of saltwater made up ahead of time, stored in a big rubbermaid tub in my garage. When the tub "inventory" drops below 10 gallons, I put more in. I mix the saltwater up in 3.5 gallons batches in a 5 gallon bucket. 3.5 gallons of DI water go in the bucket and 2 cups of Reef Crystals. Stir vigorously for about 15 seconds and dump it in the rubbermaid tub. In that tub (22g tub) is a 100W heater, and a Maxijet 900 powerhead. I keep the water at my tank temp (78 deg) and the powerhead keeps things moving and provides a nice surface ripple for oxygenation. That all runs 24/7.
My tank (46g) is in the corner of my living room, with laminate underneath it. Quick sidetrack here...
If you're putting a tank in a carpeted room, I highly recommend pulling up the carpet in the area under the tank and putting in a laminate or Pergo-type floor. That may, or may not be doable depending on where your tank will be located. But I'm sure glad I did it. It makes getting the stand on a firm LEVEL surface much much easier and I don't have to worry about the thing shifting over time as the carpet/foam compresses. It also is less of a hassle when I get messy and slop a little water down the back from my hang-on-back filter.
OK... back to the water change.
I put a big beach towel down in front of the tank, over my carpet. I go downstairs and drop another Maxijet 900 powerhead with a tube hooked up to the outlet into the saltwater storage tub. I pump 4-4.5 gallons out of the tub into a 5g bucket. I bring it upstairs. I double check the salinity, and add a calcium solution to bring the Ca level up to where I like it. I then take my orange 5g "dirty water" bucket upstairs along with my siphon hose. I siphon 4g of water out of my tank, "vacuuming" the rocks as I go.
I then put the "fresh" saltwater bucket in front of the tank, put the tube that's hooked up to the Maxijet900 (which is now in my fresh bucket) into my tank, plug the Maxijet in, and pump the new saltwater into the tank. I work the tube around my rocks, "dusting" them off.
Once I get the water changed out, I open up my hang-on back cannister filter (Magnum HOT 250) and swap out the old "blue sponge" for a new one, and give the cannister a good shake off in the old saltwater. I also take the filter from the Penguin 100 hang on back unit and clean it off in the old saltwater.
Maybe once every 4-6 weeks, I'll vacuum out a section of my sand bed, but only the top 1/2" or so. It's usually a visual thing, and not a scheduled thing.
All that's left to do is wipe off the front of the tank where I might've dribbled some water, and haul everything back downstairs and dump the old saltwater out in the laundry sink. I could probably make life a little easier by just siphoning my old salt water out the window right next to my tank, but I kind of like the plants that are below that window!
Does water get spilled? For me, I've never really spilled any in the front, but will occasionally slop some down the back. But that's just because of my hang-on-tank equipment. And also because I'm not as careful! As long as you're careful, methodical, and not in a hurry, there's really no reason to expect you'd ruin your carpet. One of the times I was in a hurry, I almost plugged in the pump to pump the new saltwater back into the tank before I had the tube in the tank... and it was lying on the floor. THAT would've been messy. But as long as you check, and double check things before you do it, you should be OK.
Regarding RO/DI water, most good fish stores will sell it. The only question you have is how well do they maintain their system? It's really just how much you trust your fish store.
RO/DI units come in all different varieties, but in a nutshell they're a combination of two types of filtration. The "RO" portion is Reverse Osmosis and uses osmosis of water through a semi-permeable membrane that lets water go though but doesn't let anything that isn't water through. Roughly. RO membranes, depending on a lot of things, gets out 95-99% of the impurities. The "pure" water goes out one tube, and the "waste" water goes out another tube. There is roughly 3 to 5 times the amount of waste water made per unit of pure water made. This water is made slowly - in terms of gallons/DAY... not gallons/minute or hour. That's how RO/DI units are rated - 100 gal/day, 75 gal/day, etc. You'll need something to collect the pure water, and a place to route the waste water to be drained away. This can be as simple as a bucket and your sink.
The other portion of a RO/DI system is the "DI" portion. This part of the system contains deionizing (DI) resin with attracts whatever impurties are left in the water and binds them to the resin. Water coming out of this part should be absolutely pure water. If it isn't, then it probably means that either your DI resin needs to be replaced.
Hope this helps. I know when I was starting out, I was playing out the whole scenario in my head and it seemed like a lot of work. Like most things, once you get a system down, it can go pretty fast.