If you can find the highest point in the room, then you can drill and fit rawplugs, then put in screws that you level to the highest point. Use a 6ft level, and you'll end up with a set of marker points that you level to the top of. Sweep the room, and be paranoid and vacumm it if neceesary, and then prime with an acrylic primer.
You need 3 to 4 people on this sort of job. One to mix, one to clean the bucket (2 of required), and then measure out the water for the next bag, one to carry, and one to level. And you can't just dump a bag of SLC in the bucket, and then mix it, you need someone to pour as the other person mixes
You add a little bit more water than recommended (maybe half a litre max), and that'll give a runnier mix. and you need spiked shoes, and a spiked roller. (okay, maybe one more person required for that job), but you pour the first mixed bag and then spread it with a float using the markers to work the height, and you spread it around, then you keep adding bucket fulls as you move down the room, and you've got the guy with the spiked shoes etc getting the bubbles out behind you. But be careful not to over roller as it starts to go off. Better to have bubbles in the floor rather than ruin it with the spiked roller.
And most importantly, you want at least 50% more bags than you anticipate using. I did this with a 3 person setup on a hot day, and ran out of bags, so had to add another layer, and it was bloody hard work, but in 2 goes, we got a 19sqm room levelled nicely.