While waterproofing with liquid latex is strong and effective, it involves coating the inside of the fabric with rubber. This process makes the garment or item both watertight and airtight on the rubber side, so be sure to consider whether this is what you really want, or whether a silicone waterproofing spray would be more suited to your purpose. Latex waterproofed fabric can be used for making everything from waterproof cloth coasters to watertight cloth bags, to Halloween or theatre costumes.
Purchase liquid latex. There are many brands and varieties available, but they generally fall into two categories: latex for theatre prosthetics, and latex designed for spreading over one's body to create customised "suits." The latter type is preferable for this project, as it spreads more evenly and comes in a wider variety of colours which you can match to your fabric (costume latex generally comes in peach). If using a stretchy fabric, bear in mind that applying latex to it will hold it in whatever stretched position you have it in during application, to some extent (latex is stretchy, but less stretchy than most knit fabrics).
Turn your fabric upside down on your work station. If you are waterproofing a garment, first wrap your dressmaking dummy, mannequin or volunteer with cling film secured with tape, then turn the garment inside out before placing it on. This will prevent the latex from sticking to the dummy or your volunteer if it seeps through the garment.
Spread the latex carefully over the inside/backside of the fabric using a paintbrush (this brush will be unusable after), or sponge. Bear in mind that some seepage of latex onto the other side of the fabric is unavoidable, especially with thinner materials. Spreading the latex more thinly can help avoid this problem, but you may find that your fabric is less watertight as a result.
Dry the latex with a hairdryer if using a volunteer (for the sake of speed). Otherwise, allow it to air-dry. The latex is dry when you can no longer move it with your finger, though it will remain slightly sticky for a while. If you wish to combat this, use talcum powder or cornstarch.
Keep drying latex away from dusty areas. Dust motes, lint, and pet hair will easily stick to it.
Latex is a common allergen. Be aware of the potential for allergic skin reaction when working with this material.