From Maangchi on YT:
Full recipe: http://www.maangchi.com/recipe/gochujang
2 pounds of barley malt powder (yeotgirem)
10 cups of sweet rice flour
1 bottle of rice syrup (1.6 kg)
4 cups (1½ pounds) of fermented soybean powder (mejugaru)
16 cups (1.6 kg) of hot pepper powder
4 cups of kosher salt
1) Mix 8 liters (32 cups) of water and 2 pounds of barley malt powder (yeotgireum) in a large basin
2) Strain the mixture and put it in a large heavy bottomed pot.
3) Heat it up on the stove for about 20 minutes until it's warm. Dip your finger in to test it: it should be warm, not hot.
4) Remove it from the heat and add sweet rice flour. Mix well with a wooden spoon.
5) Let it sit for 2 hours. The liquid on the surface will look a lot clearer, and it will taste a little sweet.
6) Bring to a boil for about 2 hours over medium high heat, until it reduces by ¼-⅓ (about 30 cups).
*tip: Stir occasionally with a wooden spoon so it doesn't burn to the bottom of the pot.
7) Add 1 bottle of rice syrup and mix well.
8) Remove from the heat and wait until it completely cools down.
9) Add mejugaru and mix well. Then add hot pepper powder and mix well. Lastly add salt, and stir until there are no lumps in the paste.
10) Transfer it to an earthenware pot or glass jar and cover with mesh or a mosquito net before closing the lid.
11) It will take about 2-3 months to properly ferment. During that time it's best to open the lid and let it sit in the sunlight during the daytime, and close it at night.