I'm glad you're smart enough not to slam tar - I cringe at how many ppl put that nasty nonsense in their veins.
How are the risks of injection tar use any worse than the risks of injecting any other form of heroin/opioid drug, other than simply how tar looks? The aesthetics of it (injecting a dark-ish liquid directly into your bloodstream) obviously aren't that great but I don't see how it's really any worse than injecting "china white", whatever the hell that is (I thought "china white" referred to fentanyl...?) By comparison the aesthetics of injecting crystal meth are great (it looks like you're simply injecting yourself with distilled water), but I'd personally argue that an IV tar habit is probably less destructive to your overall health than a regular IV meth habit. Unless there is something uniquely bad about tar that I'm missing.
Anyway, OP, the basics of injection drug use are pretty simple. Dissolve substance in desired amount of clean water (which doesn't need to be heated, even with tar, although users frequently heat tar out of sheer impatience I guess), draw up solution into a clean syringe, find desired vein and clean injection site using a sterilizing agent like isopropyl alcohol, insert syringe into vein, register vein, inject substance. You're done! Congratulations, you're an IV drug user! *horns, streamers, confetti*
But in all seriousness...do you know any other IV drug users? If so, get them to help you. I honestly would've never started IV unless someone convinced me to try it. Yes, another user actually convinced me to IV, even though I had expressed to them that I had never IV'd and really had little inclination to do so.
Actually self-teaching yourself to IV is kind of unfathomable to me...IV is dangerous and usually very intense your first time (I had used for a long time prior to IVing and I almost passed out my first couple times IVing dope) and I'd highly, HIGHLY recommend that you get someone who knows what the hell they're doing to help you. Isn't slamming a kind of knowledge that's passed down from older junkies to younger junkies anyway?
As far as "how many milligrams of heroin should I inject for my first time?": lol. Don't you realize how ridiculous that question is? For one, you didn't provide us with any background info on how big your habit is, how long you've been using etc. And there is no way any of us random people on the internet can answer a question in the form of "hey guys, I have a random street narcotic here of unknown purity, how much of it should I do?"