Cover the base with a layer of fibre blanket (or sand if the top of the base is stone or concrete.
Place the fire tiles on top of the blanket.
Place a styrofoam box on top of the tiles (=substitute for sand) where the centre of the oven will be.
Cover the box with sand building it up to form the inside shape of the oven. Place a board where the entrance to the oven will be. If you already have a door place that against the sand first.
Hold the door and the board in place with a few bricks or the like whilst finalizing the shape.
Mix the castable with enough water to make a mix that is not too runny and apply this over the sand, approximately 50 mm thick. Mix another batch and apply this over the first layer.
Leave to dry for 24 hours.
Remove the sand and the styrofoam box. Use a brush to brush most of the sand off the wall.
Cover the oven with a layer of ceramic fibre blanket (25mm), then cover the blanket with aluminium, hold temporarily in place with some heavy objects.
Ensure that the blanket is completely covered with the foil to prevent it getting wet from the render. Then cover the foil with chicken wire as a base for the render.
Cover the oven with render made by mixing one part of vermiculite, three parts of dirt and one and a half part of cement, mixed with enough water to make it into a thick sludge. Apply a second layer.
Clean it up and place the door in front or inside the entrance.
A week after starting the process, gently fire up the oven. Slow fire until the outside feels dry. When using the oven the outside wall will feel warm. Enjoy cooking!