Should I be aerating it? I mean while the biochar is in there?
you are probably overthinking it a lil, it's alright man.
Two ways to do it, either an charge with microbes as your "charge" or nutrients, in your case you are going with nutrients, and alfalfa is a damn good choice.
What I did, was similar, I used a dandelion/comfrey FPE to charge it, many people have used simply human urine for yrs and yrs in fact. I digress though.
You soak it for a minimum of three hrs and up to 3 days.
The whole concept is merely to keep the biochar from taking in your nutrients from your soil, if you "fill" it first, it has nothing left to absorb, not to mention it acts like a tiny slow release of organic nutrients as it dries.
No need to aerate.
Just soak em, I didn't put mine in a bag, I literally just filled the bucket with biochar and let it chill for a day, then strain.
If I were to do it over again I would pick a less stinky "charge" the dandelion/comfrey FPE was a little ripe...especially when I strained it... whew...
If I were you i'd throw in a handful of kelp meal too... couldn't hurt