Here's how I do beer soaps for my wholesale breweries (I currently have three).
1. Boil beer for 10 minutes. This is totally for safety reasons to get "most" of the alcohol off. Boiling for 10 minutes will remove 70-80% of the alcohol, although I do a low boil as I discovered that a rolling boil will either a) boil over the pot or b) change the natural color of the beer - so my beer might have a bit more than 20-30% alcohol. The first time I made beer soap I added lye directly to beer and it volcanoed dangerously (literally duck and run situation) out of my mixing pitcher. It was a pain in the butt to clean off my cabinets, counters, kitchen window... you get the idea. I was fortunate enough that I did not get any solution on me. I don't care for adding lye solution to anything frozen (no patience!), so I choose to boil. You can skip boiling and just make sure it is flat if you want.
2. Refrigerate overnight to cool beer. When in a hurry I have used the freezer to cool it in 4 hours. Please don't add lye to hot/warm liquids.
3. Add lye directly to cold beer. I usually do this in 3 parts, so 1/3 of the lye goes in, stir to dissolve, and repeat for the remaining 2/3 - adding 1/3 at a time in for each addition. I have added all the lye at once but I get some foaming up which scares the crap out of me, so I do it in parts. RESPECT THE LYE.
I have a separate room with a door that is dedicated for soaping, so I do not have to worry about kids/pets messing with my lye solution, so I cover the lye pitcher with a cloth and let it sit to cool to room temp. Usually this is overnight, but I have checked it at around 6 hours and it's been cool.
4. When lye solution is cooled, make soap! I do all of my beer soaps at room temp as I have the best results at that temperature. I prefer to let my soaps gel, which they do quite well with just light insulation (a cardboard box over the mold with a towel thrown over it).
This isn't the only way to make beer soap, just how I do it! I have done it also by mixing lye and water at 1:1 and adding the remaining liquid as beer after the water solution has cooled. So if you need (made up numbers for easy math) 50g lye and 125g liquid, you would mix 50g lye and 50g water (I would actually do 60g water just to make sure there is enough water to completely dissolve the lye), and then add the remaining 65-75g liquid as beer. I would make sure the beer is flat, cold, and add in parts here as well, especially if you have not boiled it.
