Concrete to wood subfloor

[ThatOneMurse]

Recently had an entry addition put on my house. I’m wanting to tile the floor from the split level stairs to the concrete entry way, I know I will have to use backer board on the plywood subfloor at the base of the stairs but how do I ensure Its level across the old doorway and into the concrete entryway? Will I need a soft expansion at the transition? And will I need to put a membrane below the tile on the concrete to keep it level all the way across? I’m planning on using ceramic tile if that helps. Thanks