Process is, first you get injection sterile water and ph.Eur/USP injection grade BA.
I am using a sterile injection water easily obtainable here (comes in packs of 20 bottles with 10, 20, 50, 100ml and so on, I prefer the versions of 20ml or more, they come in glass bottles, 10ml version comes in plastic ampules, and 10ml is anyway not much). For example, the one I am using (not sure if brands are allowed here, having post rejected all the time), a pack of 20x20ml costs something like 6 Euro, available in countless real and online pharmacies in Germany (and hence, in the EU).
BA I am getting from the lab supplies in the Netherlands. The smallest bottle you can get there is 100ml (iirc), but anything from 20-50ml or so will do the trick, and will last you forever (or at least for 2 years, until BA starts to oxidize in the opened BA bottle, and thus becomes less effective. After 2 or so years, you simply order a new one.
Then, get a sterile insulin syringes you would use for reconning, pull what amounts to 0.9-1% of BA for your bottle of sterile injection water (for example, get 1ml of BA for 100ml water bottle, 0.45-0.5ml of BA for 50ml bottle and so on.
No need to be super precise with BA amount, people are putting everything from 0.8% to 1.8% of BA and doing just fine. Also, branded BAC waters also vary in BA content `for as much as 0.2% or more from batch to batch, so no difference there.
Once you get the proper amount of BA in your syringe, put it in the sterile water, shake it around for some time to spread the BA around. That's it, you have got yourself selfmade BAC for basically pennies
As I wrote above, get your ph.Eur grade BA from the Netherlands (or from somewhere else, it's not the only place, of course), smallest bottle of 20, 50 or 100ml will last you years
It is not less sterile than, for example, reconning and filling up cartridges for the injection pens, like V1s, V2s, Savvio, GensulinPen2, Novo...