Break up and scrape out the grout from around the first tile you want to remove. Press the blade firmly to the grout and push it forward and back to dig it out. Remove it from all four sides of the tile.
Set the tip of the chisel into the space along one edge of the tile where you dug out the grout. Hold the chisel back at a diagonal angle, with the tip on the joint where the tile meets the plaster wall.
Hit the chisel with your hammer, with medium force. Scoot the chisel 2 or 3 inches along the tile and chisel it again. Repeat, moving the chisel around the whole border of the tile, loosening it, until it comes out.
Repeat the whole process for each tile that you want to extract.
Scrape off any remaining mortar or grout from the exposed wall, using a razor scraper.