I have been looking at getting a trainer in order to keep some basic fit while it is hot like this. My prosthesis is causing me huge problems with rub when I sweat heavily, and commonly why I ride most of my yearly miles in cold weather. Anyway, back on track, I was looking at a decent fluid trainer at work that was on clearance but it sold before I could get the funds together.
We have been cleaning out the shop in back and came across some rollers. I have no idea what brand, or of they are complete, but chances are good that I can get them for free before they set them out for our metal recycling guy...and that is the catch. If the owner is willing to throw them out, I cannot say what may be wrong or missing. he doesn't know much if anything about them and is really disinterested. They are too big to ebay, etc or I would BE selling them for the shop.

Currently they are apart and in what looks like three sections. There are three metal rollers and some different bits of metal frame around them. I don't see any springs, or anything that looks like a track for them to sit in or on, so I would have to assume (with my very limited knowledge about rollers) that they are the "fixed", rather than "floating" kind. Given my current money situation I am thinking about asking to have them, but am limited on room to keep them myself. I will need some sort of way to stabilize myself when I get up on them until I can learn to use them safely. If I fall over, it will be through a wall or furniture, and if I come off I will run into a salt water reef tank.

So tl;dr- How can I learn to use rollers (assuming they are complete and working) without danger of falling over and wrecking my house?

Start by riding in a doorway.

Even better, if you have a friend or s.o.,ask them to hold you (by the body, not the bike) and gradually loosen the hold.

I bet you get it in a few tries.

Oh btw it's possible the rollers are missing the belt that connects the front roller to the middle one, but that's a 5 dollar part.

and relax your upper body while riding on the rollers. Look up or forward.

It's not hard to get comfortable on rollers, but I wouldn't ever ride them near something like a fish tank. As mentioned by datlas, a doorway is a very safe place to ride them. I am very comfortable on mine, can ride no hands and all the stuff, but I'll ride off the side time to time when watching TV. Sporting events are the worst because you want to "follow" the play right or left (hard to stay centered). I can usually save it or safely clip out, but I would not chance having anything nearby.

I wouldn't if I had any other option. It's either the fish tank or my TV...the TV would make less mess...