You mix the base with an acid. If you forgot your high school chemistry, then I can run an example by you. You said your H is about 10% pure (premium stuff there, heh!). So, let's say you have 1 g of said H. That means it contains 100 mg heroin base. That is 369 g/mol. Means you have .1/369 (=2.7 * 10^-4 = 0.00027)
moles of heroin. To protonate/acidify it you need 1/3 moles of acid per mole of heroin; that is because a citric acid molecule has 3 acidic functional groups, so 3 heroin molecules can be acidified/protonated by 1 citric acid molecule. Citric acid is one of the best to use in a clandestine setting (otherwise hydrochloric or -bromic are better). Citric acid's moral mass is 192 g/mol - that means that one mole of citric acid has a mass of 192 g. Now, you have 2.7 * 10^-4 or (for ease of calculation on a computer) 0.00027 moles of heroin that needs to be protonated, which means you need just as many moles divided by 3 (see above; in reality, a little excess of like 1.1-1.2 times the theoretical amount is ideal) of acid to do the job. So, 0.00027/3 mol * 192 g/mol (citric acid molar weight) = 0.018 g. The answer is, you need about 18 mg of citric acid to acidify your heroinos and turn it into heroin citrate, aka soluble salt. Since the amount is so small, and adding excess is not a problem, you can just add like 50 mg and be fine. Hopefully that won't burn ya veins. Combining the heroin (base) and citric acid (acid, duh) in the right ratio should leave you with a solution that is roughly neutral - excess citric acid will make it more acidic, so don't go ham with it, cause otherwise your veins might not appreciate it.
OT: morning beer and about to take 600 mg codeine.
E: I hadn't realized it, but since your heroin is only, at best, 20%, then what is the rest of that shit? If it's something that can be protonated (think alkaloids), then you might need more citric acid to turn all the heroin into heroin citrate. Although 50 mg acid per 1 g crude/stomped on heroin should do it.