I have been asked to do a tiel job. The problem is that there is a floor drain that doesn't slope properly. I need to put a small slope on the floor but not too much. The existing floor is concrete. Do I use a Mud bed? How do I get a gradual slope over a large area? Please help.

A mud floor would be your best bet. Determine how much slope you need. Chalk lines along the wall as a starting (high) point then slope the mud toward the drain.
You can use a 6' level and a long striaght 2x4 as a screed board.

There was a tile forum I think member Dherman mentioned a while back, but I forget what it was.