If you're really set on doing this, my recommendations are similar to others posted here:
* get a milligram scale and start with a *very* small dose, such as 5mg.
* don't re-dose too soon. come-up from non-IV ROAs should happen within 30 minutes. don't let that shit sneak up on you by re-dosing too soon.
* don't mix substances. especially avoid mixing with other CNS depressants (such as benzos or alcohol). but ideally, don't use anything else where you're learning
how your body reacts to H.
If you're getting #4 heroin (which you probably are if you live in the US, east of the Mississippi), I would recommend snorting instead of smoking. It's easy to lose dope due to poor 'technique' if you're smoking, and this loss interferes with keeping track of how much you've done. If you insufflate (snort) it, you won't get the highest bioavailability, but you will know exactly how much you've consumed.
I'd feel bad if I didn't offer a shout-out to skipping opiates, though. Dude, I was in your shoes about four years ago. I've been trying to quit for about two years now, and it's been the most awful experience of my life. I never thought heroin addiction would happen to me, but it did. It's not glamorous, and it's often miserable. The addiction sneaks up on you; it's incredibly hard to recognize it as it gets out from under you.
But, I read a million posts on BL saying not to do it, and I did it anyway, thinking a) I was strong enough to handle it, and b) my life was so fucked up that I didn't care if I did get addicted. So if you do move forward, go slow and be extremely careful.
Good luck.