Unfortunately, it is quite a bitch. Tar is a very crude blend of 6-MAM, DAM, Morphine etc, and many inactive and or partially-unreacted alkaloids. It is a resinous material that is NOT suitable for inject. Aside from sterility, you are injecting a sludge which may contain plant particulates and various things that should never be injected. Nonetheless, I have injected my fair share of this stuff, and was never really able to "clean" it up to a point where it is not sketchy. There is one very low yielding way to do it, using alot of tar to get one injection of micron filtered solution that is very light yellow and translucent, but again, you have to "waste" quite a bit to yield a solution of this caliber, and is not practical. I also tried some methods dissolved in various solvents in a centrifuge, as well direct attempts to isolate certain salts (not so easy), and various extractions and re-precipitations, washes etc etc. Logistically, this is far too expensive and impractical for heroin users.
Aside from this, there are less complicated "safer" way of injecting tar as is, but you will again need to "waste" some tar in the process. Yes you can shot it up with a crude cotton filtration and you will get "more for your money" in terms of effects, but you may pay the price later on.....
And there is little to NO uniformity with tar heroin. Batches range wildly in content, some of the crumbly dry tar is usually the easiest to purify. Soft, gummy and pliable tar is the worst quality, and should be avoided. A "decent" tar heroin specimen is generally very dark, harden chunks similar to a harden candy, and is difficult to cut, and can be shattered/smashed. The best I can really offer is, use bacteriostatic water ALWAYS, fresh syringes, and based on your budget, micron filter it using alot of fluid multiple times.
Also, to sterilize, dissolve the tar in a rather large amount of pure ethanol (if you cannot obtain it, you can use176 proof everclear - not homemade, the legal bottled kind). For example, for .5 grams (a typical balloon size), use 10ml of the ethanol to get a diluted solution, let it sit to kill various pathogens, run it through a a 1.2 micron filter a few times, then a .8 micron, and finally allow the ethanol to evaporate. This will not necessarily kill all pathogens, but it will kill quite a broad spectrum After you have done this, let it remain in the suspension until the ethanol evaporates (I use a hotplate at low temp). After this, add 3cc of bateriostatic water to the now dry and cleaner heroin. I used to use the high setting on the hot plate (this was all done in pyrex) for a few seconds to flash sterilize it further. This could be option I suppose. If you do not have access to 5cc syringes with proper screw on needles and only have insulin syringes, add perhaps 2cc instead of 3cc and very slowly at low heat, evaporate the excess water.
Suspended in bacteriostatic water, run it through a .4 and then .2 micron filter, and you will have a "safer" tar heroin.