Place the chisel at the top of the tile where the trim meets the plaster and hammer down along the edge. These pieces should break off fairly easy and give you access to the mortar. Chisel into the mortar in the gap between it and the plaster. You are trying to clear an area of mortar back to the lath and studs.
Place the pry bar against one of the studs you have exposed and pry the lath away from it. Work from the top of the mortar down, finding and prying at the point where the nails attach the lath to the stud. Using work gloves, grab the mortar and lath and peel it down away from the wall as it breaks away.
Chisel the mortar and lath from behind as you expose and pull down sections to break it up into smaller pieces. This will help with removing the lath and mortar from the room. It should come off in sections of lath with the mortar and sometimes the tile still attached.
Break the tile and mortar at the bottom where it meets the floor with the hammer. This will separate this mortar from the mortar that was likely used under the floor tiles.