There’s a trick most campers know about, where you can spray some PAM or nonstick spray on your tent to help the tent push off any rain that might fall. Although there are far better sprays to help with this. The concept of waterproofing has been in practice for thousands of years, however.
The most common way it was done then was finding leaves that were so smooth that water merely rolled off of them. This means you can find big leaves like this in the wild and use them to cover your shelter. Some cupped versions can be good to help lead water into a certain area, allowing you to collect the rainwater for much-needed hydration.