I'm about to tile my bathroom floor but the floor is concrete and it is not flat. It still has the thinset trowel markings on certain areas and they are not in the same direction. How do I even out the floor?
Thinset mortar is usually fairly soft. For that small area you can probably knock down the lumps with a belt sander and coarse grit belt. You don't need to entirely remove the old mortar if it's well bonded. Just get a fairly flat top. Vacuum thoroughly before proceeding to remove all dust and debris.
Since you're presumably going to use a 1/4" notch trowel to set your tile, the mortar will accommodate the remaining roughness. It'll be a bit more difficult to apply the mortar than on fresh concrete, but no big deal.
I had the same scenario with my floors in the house we just moved into. I had a lot of the areas ground down with a floor grinder, but there were a few areas that needed attention like yours. The man doing the grinding suggested I used his 4" scraper, he used it all the time for areas like yours and what I had, I though he was joking to a degree or had a lot more strength to pull something like that off, but I tried it anyway and was completely taken by surprise how easy it came up.
It is a small investment, and a dust free way to get a small area such as yours clean to the concrete surface.