I'm working on the set for "Into the Woods" and started with Cinderella's fireplace.
Step 1: Big Sheet of Styrofoam Insulation
I started with a 4x8 panel of insulation styrofoam found at Home Depot. I cut it to a 4x6 piece(using the extras for the sides) and sketched out the front. I carved the brick shapes and peeled the plastic covering off. After cutting out the opening, I carved smaller bricks for the back of the fireplace.
Step 2: The Power of the Heat Gun!
So, this is the cool part....if you melt the foam with a heat gun, you can soften the edges and melt the styrofoam appearance. This give you the brick and stone texture also.
Step 3: Base Coat the Foam
Stick with water based paint...spray paint will melt the foam. I used flat latex interior paint. Then faux finished the stone, brick and wood.
Step 4: The Mantle
This stuff can even be faux wood! You just carve and melt to your desired effect!
Step 5: Back and Front
I used black card stock and gorilla tape to hold the pieces together. The whole thing looks like it weighs tons but is only about 5 lbs.