Start by setting your 8 gallon stock pot on the stove. If you have multiple, different sized burners on your stove top, choose the one with the most powerful flame.
Next, add water to your pot. Water chemistry becomes a bit more important when you are actually mashing your grains as opposed to just steeping them, so it is a good idea to get your water set out the night before you plan to brew. This way the chlorine in the water will dissipate out naturally, leaving you with a better tasting finished product. A general rule of thumb for 3 gallon BIAB batches is to start off with 5.5 gallons of water. The grain will absorb some water during the mash, and you will also be boiling off about 1 gallon over the course of the 60 minute boil. If you end up a little under 3 gallons after the boil, you can always top up with more water to hit the 3 gallon mark.
Once you have your 5.5 gallons of water in the pot, turn on the burner to begin heating the water. You can place the lid on the pot to help heat the water a bit quicker. In general, you want to heat the water about 12 degrees higher than where you want the mash to rest. So, for example, if your target mash temperature is 152F, heat the water to 164F before you begin adding your grain. This temperature is called your strike temperature.
Once you hit your strike temperature, turn off the flame and place the mesh bag into the pot. Make sure the open end of the bag is wrapped around the rim of the pot so that you can hoist it out once your mash is complete. Be careful not to touch the pot or the water as it is very hot.
Once your mesh bag is in the pot, begin to add your crushed grain (the crushed grain is also referred to as the grist). Make sure it all gets into the bag to keep all the grain material contained. Stir as you add the grain, or add grain in small increments and stir so the grain is well mixed into the hot water. The goal is to break up all the clumps (dough balls) that may form when adding the grist to the water. If you don’t break up all these clumps, you will miss out on all the good stuff (starches and sugars) inside of that clump.
Now that all of your grain is thoroughly mixed into the hot water, take a temperature reading with your thermometer. If you have hit your mash temperature (or are within a degree or so), then you can place the lid on the pot and start your timer for 60 minutes. If your temp is too low (say 3 degrees or more below target), turn the flame back on, but make sure you stir frequently while you heat the mash with direct flame. If you do not stir while the heat is on the mash, you could scorch your grain, resulting in harsh, astringent flavors in your beer. If your temp is too high, you can add cold water in small increments until the temp is within a degree or so of the target. Add no more than 2 cups of cold water at a time, and stir in each addition to get a good reading of the temp of the entire mash.
Place the lid on the pot and let the mash rest for 60 minutes to ensure all of the starch in the grain is converted to fermentable sugar. Every 15 minutes or so, check the temperature of the mash to make sure it is holding. Again, if the temp drops more than 2 degrees below our target, you can turn the heat back on and stir frequently to bring the temp back up to our target.
After the mash has rested at the desired mash temperature for 60 minutes, it is safe to assume that the mash is complete. At this point, carefully lift the bag (it’s very hot and can be heavy!) and all of it’s contents (the grist) up out of the liquid and place your strainer across the kettle. Try not to spill much of the liquid outside of the kettle when doing so. This liquid contains the sugar we have just worked so hard to create. Rest the bag containing the grist in the strainer and allow the liquid to drain through the strainer and into the brew pot.
At this point, you can begin heating your wort (the sugary liquid we have just created) up to a boil while the residual liquid in the grist drains down into the pot.
Carry on with the brew just like you would for an extract batch. Add hops at designated times, chill your wort once boil is complete, transfer wort to your fermenter, pitch your yeast and for fermentation to complete!