You can use any number of things: muslin (rinse it thoroughly to remove the sizing), hair canvas as Rory does, self-fabric. In dressmaking, some people use white silk organza because it can be seen through.
I use a piece of hair canvas, same as you would for interfacing the front of the coat, with the edges overcast. It's good because it's slightly springy and so it is more inclined to lay flat without wrinkles like cotton etc. would.
You can use any number of things: muslin (rinse it thoroughly to remove the sizing), hair canvas as Rory does, self-fabric. In dressmaking, some people use white silk organza because it can be seen through.
I use a piece of hair canvas, same as you would for interfacing the front of the coat, with the edges overcast. It's good because it's slightly springy and so it is more inclined to lay flat without wrinkles like cotton etc. would.