How to Get Sulfur in the Sulfur and Potassium Mod
Mining Sulfur Ore
Sulfur Ore generates in the Overworld and drops Sulfur when mined. Use any pickaxe and collect the drops directly. Sulfur Ore is your standard source for Sulfur during early play.
Mining Sulfur Nether Ore
Sulfur Nether Ore generates in the Nether and also drops Sulfur. Bring a pickaxe and enough supplies for a Nether trip. If you already have Nether access, Sulfur Nether Ore is a second source of the same item, so you can pick whichever location is closer to your base.
What Sulfur Is For
Sulfur is a crafting ingredient. Once you have Sulfur in your inventory, check your crafting recipes for shaped and shapeless recipes that use it. You can also store Sulfur as Sulfur Block: craft Sulfur into Sulfur Block to compress your stockpile, and break Sulfur Block back down when you need loose Sulfur again. Sulfur Block is a storage block, so it is useful for saving chest space when you return from a mining trip with a full stack.
Practical Route
If you are already in the Overworld, mine Sulfur Ore first. If you have a Nether portal ready, Sulfur Nether Ore gives the same Sulfur drop. Bring the Sulfur home, craft it into Sulfur Block for storage, and keep a stack of loose Sulfur for crafting.