Please don't do it especially if it's black tar.
Skin popping or subcutaneous injection
(SC) of illicit heroin
is a bad idea and it's worse for the likes of black tar or brown heroin which requires an acid (and hurts like hell). SC or IM injection of anything other than sterile made for injection preparations is going to cause an abscess and will lead to infection, maybe not the first time, maybe not the second time but it is
I highly recommend you do not peruse this Route of Administration. By injecting in to a vein, any pathogens or microbes will be dealt with promptly and effectively by the immune systems army of white blood cells giving them no time to take hold. In contrast injecting in to a muscle or just under the skin also known as muscle and skin popping (IM and SC) will allow any potential pathogens, namely bacteria, to take hold and start multiplying.
I can't advise against it strongly enough. If however you are getting white heroin 4 and could acquire some fine micron wheel filters which can, if small enough, filter out bacteria then that would be the only safe way really. I don't know much about black tar but I do know it's can be relatively thick and I doubt (but can't say for sure) that a fine wheel filter would be effective because I suspect it would simply get clogged up. As I said I don't know for sure and if you're going to do it anyway then it's certainly worth trying.
If you want to know what skin popping black tar will do to you, search "skin popping black tar" on Google Images and I warn
you, it is not for the squeamish.