Please remember that unlike the medications prescribed to you after surgery, heroin is not going to be pure. Street heroin is going to be cut with other substances that may make use more risky. For instance, the addition of more potent drugs like fentanyl can drastically increase the chance of overdose. Since it sounds like you don't have a reliable source, I'd proceed with extra caution. Also, if you're used to taking opioids orally, then keep in mind that these other routes will be faster/ have a stronger come-up. Nausea is a definite possibility (honestly, almost a certainty).
If you're going to use heroin, then I would suggest snorting or smoking it to injecting it. I guess that'd be more dependent on if you got powder or black tar. Snorting-wise, the "starting" dosage I've been referred to is 5- 20mg, which is supported by
the psychonaut wiki. But it'll depend on the person's opioid tolerance and individual factors (e.g., size and genetics).
I'd definitely recommend trying it with someone else around that has naloxone (as someone else already said). Do a small test dose and wait a few minutes to figure out purity and strength. Also don't combine it with another sedative (alcohol, other opioids, benzos, barbs, antihistamines) since that'll increase the chance of respiratory depression/ overdose.
Hope that helps