There's no way to know exactly how much H you'd need if you bought it off the street. Dealers cut it with so much shit it's a shot in the dark. These dealers probably mix the least amount of H as possible. They're in it to make money, not have their customers drop like flies. You'd need a good bit I think. I don't know anything about the dark web so I couldn't say to try that unless you're comfortable with the risk. But it's supposed to be more pure.
Mix just enough water so it can push through the tip of the syringe. Mix a little at a time. You don't want it too watery, but not too pasty either. The size syringe is relative to how much H you plan to use. 1 or 3 ml maybe. 5 at very most.
I guess the only thing then is stick the syringe an inch to an inch and a half up there. Maybe wrap some electrical tape around the syringe where you need to stop so you can feel where you're at instead of guess, get comfortable, and see what happens. Benzos and alcohol to go with it wouldn't hurt.
You can also do it vaginally but I have no clue as to how far the syringe needs to go. Maybe the same length as rectally, but don't quote me.
Hope this helps.
