⚡ Quick version (for the impatient)

Get a 50 ml bottle of pharmacy sterile water (e.g. Ampuwa).

With a fresh syringe, draw up 0.45 ml pharma-grade benzyl alcohol (Ph. Eur. in Europe, USP/NF in the US).

Add it to the water bottle and shake well for about 30 seconds until no oily streaks are left.

Different bottle size? Use 0.09 ml benzyl alcohol per 10 ml of water (20 ml → 0.18 ml, 100 ml → 0.9 ml).

What BAC water is, the alternatives, the shelf-life data and how to reconstitute a peptide are in Reconstitution 101: BAC Water and How to Reconstitute. This thread is only about mixing it yourself.

Details, the maths behind the 0.9 %, how long your batch keeps and where to buy in Europe below. 👇

1. What you need

Sterile water: any pharmacy-grade sterile water works, e.g. Ampuwa® (Fresenius), Aqua ad iniectabilia (B. Braun). Ideally in a bottle or vial with a rubber stopper you can pierce. If yours comes in glass ampoules, transfer it into a sterile empty vial first. Not suitable: distilled, deionised or tap water, none of these are sterile.
Benzyl alcohol (BA): pharmaceutical grade, purity >99.95 %. In Europe and the UK look for Ph. Eur. (or BP) grade, in the US for USP/NF grade, usually sold as "Benzyl Alcohol NF". Both standards are equivalent for this purpose.
Syringes & needles: sterile and strictly single-use. A 0.5 ml U-100 insulin syringe is ideal for 50 ml batches: 0.45 ml sits exactly on the "45 units" mark, so you read one line instead of estimating between two. For other amounts a 1 ml syringe with 0.01 ml graduations works.
Alcohol swabs for the rubber stoppers

2. What "0.9 %" actually means

Bacteriostatic water contains 9 mg of benzyl alcohol per ml ("0.9 % w/v", the pharmacopoeia definition).

Benzyl alcohol is a little heavier than water, so 9 mg are slightly less than 0.009 ml. In practice:

Quick rule: water (ml) × 0.009 = BA (ml). Lands at about 0.93 %, which is fine.
Exact: water (ml) × 0.0087 = BA (ml). Hits 0.90 % on the dot.

The difference is smaller than what you can read on a syringe, so either rule works.

3. How much benzyl alcohol for common volumes

Water    BA exact     BA mass   Quick rule
(ml)     (ml)         (mg)      (x 0.009, ml)
  3      0.026          27        0.027
 10      0.087          91        0.09
 20      0.174         182        0.18
 50      0.434         454        0.45
100      0.869         908        0.90

A 1 ml syringe covers every amount in the table, so you can always draw the benzyl alcohol in one go.

4. How to actually hit 0.9 %

Batch size matters. A 1 ml syringe can be read to about ±0.005 ml. At 3 ml of water that is an error of roughly ±20 %, at 50 ml less than ±2 %. So rather make 20–100 ml at once than tiny batches.
Use the smallest syringe that fits the BA amount (a 1 ml syringe for everything in the table). Draw slowly and push out all air bubbles before reading the scale.
Acceptable range: anything between roughly 0.85 and 0.95 % is fine. Much less weakens the preservative effect, much more does not help.

5. Steps

Wipe the rubber stoppers of the water bottle and the BA vial with alcohol swabs and let them dry.
Draw up the BA amount from the table with a fresh sterile syringe and remove all air bubbles.
Add the benzyl alcohol to the water bottle through the stopper.
Shake the bottle well for about 30 seconds until no oily streaks or droplets are visible.
Label the bottle with the date and "0.9 % BA" and keep it in the fridge.

About shaking: Shaking is fine here and actually helps. Benzyl alcohol is an oily liquid that dissolves in water up to about 4 % (PubChem), but only if it is mixed properly. Swirling gently can leave droplets floating on top, and then the concentration is uneven. The "never shake" rule is for peptides: once a peptide is dissolved, shaking can damage it, so there you swirl or roll gently. Plain bacteriostatic water has nothing in it that shaking could harm.

6. How long does your batch keep?

Handled cleanly, use homemade BAC water within about 90 days of mixing. The measurements behind that number are in Reconstitution 101.

Small overfill as a buffer. If you plan to use a bottle for longer, go slightly above 0.9 %, e.g. 0.5 ml instead of 0.45 ml per 50 ml (≈1 %). Commercial pharmaceutical preparations use benzyl alcohol at 0.9 % up to about 2 %, so 1 % is completely normal. Much more has no benefit, and at higher concentrations benzyl alcohol can make some peptides clump.

Discard earlier if the water turns cloudy, shows particles or flakes, or smells clearly of bitter almond.

If you only need a few ml per month, simply mix a fresh bottle every three months and throw away the rest. Sterile water costs about one euro, so splitting a bottle into small vials is not worth the contamination risk.

7. Where to buy in Europe

Sterile water: in any pharmacy, usually without prescription. The product name is "water for injection", in Germany/Austria "Wasser für Injektionszwecke" or "Aqua ad iniectabilia", in France "eau pour préparations injectables (EPPI)", in Italy "acqua per preparazioni iniettabili", in Spain "agua para inyección", in the Netherlands "water voor injectie". Common brands are Ampuwa® (Fresenius, e.g. the 50 ml "Frekaflasche") and B. Braun. Online pharmacies stock it too. Watch out: irrigation solutions ("Spüllösung", "for irrigation") are a different product, even from the same brand.
Benzyl alcohol Ph. Eur., Germany: many pharmacies do not know the product. Give them the PZN (German pharmacy product number), then any pharmacy can order it:
- Caelo Benzylalkohol, 100 ml: PZN 10770897 (around 53 €)
- Hecht Benzylalkohol: PZN 17556405 (around 30 €, the cheaper option)
Benzyl alcohol online: Laboratoriumdiscounter (NL), 99.95 %, FCC/USP/Ph. Eur./BP, ships across the EU. Lab suppliers such as Merck/Sigma often sell only to businesses.
Syringes, needles, alcohol swabs: pharmacy or online medical supply shops.

Sources

PubChem: Benzyl alcohol (density, water solubility about 4 %)
Shelf-life sources: see Reconstitution 101

Part of the Grey Market Survival Guide, chapter 5.3.