You just put the brick into a 5 gallon bucket and add water from the hose. It doesn't matter how much water, because once the brick is all fallen apart, you're going to dump the water off.
Cold is fine - it wil lsit for the 30 minutes so it will be cold by the time you place it in the enclosure - you will want to wring out alot of the excess water and let it air dry for a bit
Cold water will work, but hot water works SO much faster. Long ago I ordered a few bricks from a terrarium supply site. I put a couple in a bucket of cold water, and even let it sit overnight. I still had to manually break them apart and it took forever. Then I was left with a soupy mess at the bottom.
Next time I dropped a couple bricks of the same stuff in a bucket I filled with boiling water (1 650 gram brick to 1 gallon of water), then covered it to trap the steam. Within 10 minutes the bucket was filled to the top with fluffy coir and hardly any excess water. All I had to do was dump it in the enclosure and spread it around.
How the brick breaks apart may depend on the brand, but always use hot water now.