Note: "The stuff" can be acquired at the Trouble Brewing minigame; each game lasts 20 minutes and will get you 100 pieces of eight, assuming depositing 100 resources. This buys two "The stuff", which is essential to making money from this method.
There are two locations to brew ales at: upstairs of the Laughing Miner Pub in Eastern Keldagrim and the basement of The Green Ghost inn in Port Phasmatys. Start with the vat in Keldagrim.
Fill your inventory with two empty buckets or buckets of water, two barley malts, one package of "The Stuff", four clean harralanders, two empty pots or pots of ale yeast (there is a pot respawn in Keldagrim), and eight empty beer glasses. Head to the vat in Eastern Keldagrim via the trapdoor north-west of the Grand Exchange for the quickest access. Alternatively, you can use the Blast Furnace minigame teleport. The Laughing Miner Pub is located north of the mine carts, and north-west of the pub is a well to fill your empty buckets with water if they aren't already filled. Head to the pub and go upstairs to the brewery. Pick up two pots from the pot respawn near the vat if you don't have them already. Talk to Blandebir to fill up your two empty pots with ale yeast for 25 coins each. After you have all the necessary items, fill up the vat by putting in the two buckets of water, two barley malts, The Stuff, four harralanders and one pot of ale yeast. You must fill the vat in the correct order—don't forget The Stuff, it increases the chance of mature ale, which is worth more, from 5% to 64%. After you put the ale yeast in, the ale will begin to ferment. The vat in Keldagrim can take eight hours to two days to finish fermenting.
Fill your empty buckets at the well again if you need to. Head back to the Grand Exchange via the mine carts, and withdraw the same items once again to fill the vat in Port Phasmatys. Withdraw an ectophial for the quickest access to the brewery, two barley malts, one package of "The stuff", four clean harralanders, eight empty glasses and two buckets of water if you don't have them already. You should have one pot of ale yeast in your inventory from your earlier trip to Keldagrim. Use your ectophial and head to The Green Ghost Inn, south-east of the Ectofuntus. Go downstairs to the brewery, and repeat the vat-filling process by putting in the two buckets of water, two barley malts, The Stuff, four harralanders and the last pot of ale yeast, in that order. After you put the ale yeast in, the ale will begin to ferment. The vat in Port Phasmatys takes a minimum of two days to ferment, but rarely exceeds that.
When a vat of greenman's ale
is done, the examine text will say "This vat is filled with [mature] Greenman's ale."
Click the valve on the vat to empty the vat into the nearby barrel, and then use your empty glasses on the barrel to collect the ale. If you check the Port Phasmatys ale two days after it begins fermenting to see if it's done, you can go through the same vat-filling process to collect and refill the ale vats, starting with Keldagrim, in one go.