I am currently remodeling a 3,000 square foot house. We are laying tile in 1300 square feet of this house, including the front room, bonus room, kitchen and two dinning areas. Also an entry way, which connects the bonus room and the living room. The entry way is 15 x 8 feet, unfortunately the entry way is almost two inches lower than the rest of the house, WHY, we haven't a clue. This entry way also joins the living area of the house with the hallway to the bedrooms and a entry way closet.
#1 Clean the area thoroughly, including sweeping and vacuuming. Remove all debris and make sure it's not wet.
#2 They lay down a felt style paper, this is mainly a moisture barrier, making sure to use plenty of staples.
#3 Next is lay, a metal mesh, I call it chicken wire. With the paper and wire it is important to cover the complete part of the floor that is being leveled. Once again use a fair amount of staples with the wire, you want the paper and wire to be secured.
#4 Here comes the mud.............. The mud is what will level out the floor, and later harden so you can tile from the wood to the mud covered entry way seamlessly. They usually use a large tub in the front yard to mix the mud. carried in one bucket at a time, until they have enough to fill the void.
#5 Leveling the Mud, this is most important. But in this video I try to cover how professional packs and levels the Mud. Using various tools, 2x4s, a 6 foot level and years of experience.
I included a video so you can watch how a man with years of experience attacks the void by manipulating the mud until it has covered the entire area............... and level
Good luck with you DIY project.