Choose a sunny day to do make flame retarded cloth, so that you can dry the cloth in the sun or on the porch, rather than having chemicals dripping inside.
1In a large pan, mix 1 lb of alum with 1 pint hot water from a faucet. A preserving pan is a good size for this, to allow room for the fabric.
2Select the fabric that you wish to treat. Dip it into the pan and wet it completely.
3Pull out the completely wet fabric. Put into a non-drip plastic basket to take outside. Hang on a line, over the basket, or over a clothes rack.
4Use once dried. Expect some stiffness compared to the original fabric but it should mold to the shape needed with bending.
Ammonium chloride and ammonium phosphate formula
Another borax variant
Sodium silicate formula
National Fire Protection Association formula
Another variant on the borax methods.