Sulfur is the brimstone of the Bible and has been known for thousands of years. Sulfur atoms are pretty common and occurs in many minerals. The most prized for sulfur extraction is pyrites (fool's gold).
Despite its abundance in nature obtaining useful sulfur (somewhat pure sulfur)is going to be a big hurdle for most people post SHTF. Sulfur has many uses and making gunpowder is the big one! You can make gunpowder (black powder) without sulfur luckily, but the black powder that uses sulfur is more powerful than black powder without it and easier to ignite.
The only easily extracted sulfur in a SHTF scenerio will be at certain volcanic vents and a few hotsprings. So although this site is about knowledge that will be beneficial in the far future it is such a huge advantage to have that I would stress the stockpiling of sulfur to get you and your group through the early decades of total societal collapse. Do not use your sulfur for hunting purposes and such. Reserve this best powder for self defense.
Most people will have access to sulfur through the mineral pyrite. Pyrite is not pure sulfur, but instead a iron and sulfur compound that you refine into pure sulfur.
Pyrite also goes by the name Fools Gold because it looks similar to gold. Anyone that has done any gold panning can easily tell the difference simply by looking at the color of the mineral if it is pyrite or gold. Pyrite has a darker yellow color to it that's similar to brass, while gold has a brilliant yellow color that is very reflective.
Shape can also be used to help identify pyrite. Nearly all pieces of pyrite are formed into the shape of crystals.
You probably know that gold is one of the heavier metals. Panning for gold relies on this property of gold. Pyrite weighs less than gold, and small flakes of pyrite will wash away when placed under the water.
To extract the sulfur from pyrite (53% Sulfur) or other rocks such as Galena (13% Sulfur), can be done using a two pot distillation method. To do this obviously you will need two pots. The pots were traditionally made of clay and you can use modern or clay pots. Your choice. The two pots are connected by a short pipe. The crushed rocks are placed in one pot. This pot is heated. The sulfur would evaporate and the fumes would be flow into the other pot via the pipe, where it would cool and condense. Any slag (mostly iron) left in the first pot can be discarded or used if you have a use for iron.
Be aware that there is a real danger H2S which is highly toxic.
Scale up the process as you resources and needs dictate. Historically batches of hundreds of pounds or more were processed at time.