Tiling over Concrete Basement floor

I'd like to tile over my concrete basement floor in an area about

I've done plenty of tiling on thinset over wood but the surface has always been flat, give or take very minor amounts. I'm concerned that although the floor is relatively flat as you move along it that it varies in slope a bit here and there, maybe an 1/8" or so.

How do pro's do a job like this? Should I just ignore the possible major variations from end to end and level the tiles as I go with the thinset so that it's level along the way, but possibly not perfectly level from one end to the other?

Also, what should I do to the concrete before using thinset? It's about 20 years old, not too dirty but not exactly washed clean. What's needed before I tile?

Thanks,