You can do it either way Green Bandit, you can sprinkle it on top and water it in or mix it. I personally like to mix mine but there is a catch to it, it doesn't dissolve well. What I do is take a quart jar and add the Tspoon of aluminum to it. Get your water hot, very hot, fill the quart with it and leave some head space. Shake the living shit out of it! You still won't get it all mixed but you will get most of it.
Have your pH'd water ready to go and pour the quart into it so it cools the water down a bit. If you have 5 gallon equiv airpot then you will probably want to use maybe 1.5 - 2 gallons total. The thing is, you want to distribute that water with the aluminum in it through all of the soil and try not to get any runoff. You want it to stay in their.
A couple of warnings, DO NOT breath the dust of that aluminum sulfate! DO NOT breath the air of the quart jar when you open it, there is going to be some pressure behind it. It will take your breath, trust me, I know from smelling it (like an idiot). Lastly, see my last warning, the aluminum reacts with something in tap water and begins to build pressure so don't fill that jar and walk away from it. Be ready to pour it out when you have mixed it good. I haven't had any problems but I have noticed that it builds some pressure, not a lot but enough that I thought to myself I wouldn't leave that sitting there for an hour.
I would start with 1Tspoon and see how that does for you. You may need to add a little more throughout the next few weeks to keep it down and that wouldn't be surprising. The main thing to remember is to keep the soil moist. Don't believe the BS that you read about wet/dry cycles. Small steady amounts of water is better than adding gallons then letting it dry.