Linseed oil and waxing were two very easy ways to "waterproof" fabrics. However, everything comes with a down side. Just like today, once you official waterproof something water cannot get in, but it also cannot get out. This makes it less than desirable for things like clothing on the body, tents, etc. Oiled silk is advertised for umbrellas in the 18th century as an example of normal use.

Most of the time people relied on the natural water resistance of certain fibers and weaves. A tightly woven and fulled wool broadcloth (or many other wools for that matter) can be incredibly repellent of water. It can absorb quite a few times it's weight before feeling damp and much of the water rolls off anyway. Outer garments like cloaks or great coats are usually made of this material. Textile goods like tents rely on the weave of the linen or hemp canvas which actually creates small channels in the weave to encourage the shedding of water. Once pulled relatively tight it takes a very long time to leak (and usually leaking is due to touching the surface from the inside).

There is a surviving soldiers knapsack from the 1770s that is painted with a linseed oil based paint. It renders it waterproof and keeps the soldiers goods dry. For a while at least. The original has lost most of it's paint over the years, it's still there between the woven fibers. Reproductions of this item always last for a short while before the paint begins to crack and crumble off at the joint for the flap. A single crack and now it's not really waterproof at all. Clothing, with all of it's creases and manipulation, would cause the paint or wax to crack in time. This also often causes the fabric itself to crack in the same place, ruining the garment.