One tip for making fabric waterproof is to rub warm beeswax on the cloth. Another tip is to wipe the fabric down with linseed oil.
When using beeswax to waterproof fabric, choose a pure beeswax that doesn't contain any additives. Heat the beeswax and the cloth with a hair dryer or by leaving them in the sun for several minutes. Make sure the fabric doesn't get too hot, and don't melt the beeswax. Rub the beeswax over the cloth, moving in both directions to get the wax into the weave of the cloth.
Make the beeswax even by spreading it out, and rub it into any seams or corners. Next, use a hair dryer to heat the cloth again to melt the wax into the cloth. This process makes the fabric a bit darker. Then, smooth any pools or uneven areas by rubbing in circular motions with fingers.
Now, let the cloth cure in a warm and dry place for 24 hours. The fabric is a bit stiff after curing, but softens again over time. However, the darker tint doesn't go away.
When using linseed oil, stretch the cloth across an open back frame and secure it with clips. Use a rag to apply a coat of the oil and then wipe off residue after 30 minutes. Let it dry for 24 hours and repeat this process.