I have this plain hallway that I wanted to dress up with some faux columns
I started with some 5.5" MDF and then sent it through the table saw with the blade at 45 degrees so the two corners would meet at a corner.
The corner moulding is 3/4" wide, so I measured where the baseboard and crown would be and then added 3/4". I cut some 1/4" pieces of plywood to put at the top and bottom with that measurement so there would be 3/4" all the way around once the baseboard and crown were on. I measured the baseboard and drew on the baseboard where I needed to trim with the multi-tool and chisel.
Then I nailed it in place
I cut and put the crown in place, filled with filler, sanded, and caulked all around the whole column.
Ta-da!