Many nylon products are already waterproof when you buy them, as many manufacturers give the synthetic fabric a coating that makes it resist water, particularly if you're buying a nylon tent or coat. However, if your waterproofing has worn away over time, or if your nylon product didn't come with waterproofing, there are several different sprays you can use to ensure your product's water resistance. The spray is, after all, a lot less expensive than a new tent.
Make sure the fabric's colour won't run if you apply a particular waterproofing spray. This may be more important for a garment than for a tent. Find an out-of-the-way spot on the nylon and spray some. Use a paper towel to wipe the spray into the fabric, and check to see if any colour leaches onto the towel. If the colour does come away, you will ruin the look if you continue.
Wash your item thoroughly. If it's a tent, put soap in a spray bottle that connects to your hose and clean the whole surface. Then take the sprayer off and rise the tent with water. If it's a garment, throw it into a washing machine set for warm water. Don't throw it in the dryer; hang dry it.
Drape the item over a workbench in your garage, or lay it out in your backyard, and prepare to spray the waterproofing liquid. Put on a face mask to keep the fumes out of your lungs. If you're working with a garment, a normal clothes hanger will work.
Apply the waterproofing spray, making sure to completely cover the seams, until the whole garment or tent has an even coating. Allow the spray to dry to the point of being tacky to the touch.
Use heat to seal the waterproofing to a jacket. You can use a steam iron set to high power, or throw the item into a dryer set on high. Do not use the item until this process is complete.