Hi there.
Yes, Heroin can be administered rectally and it has relatively good bioavailability depending upon what reference you consult. My understanding after years of both research and personal experience, is that rectal administration is likely equipotent or ever so slightly less potent than administration via insufflation. This bioavailability marker is generally tacked at 66% or 2/3 whereas both vaporization (smoking) and injection both possess a ~100% bioavailability.
If you are used to vaporizing, you may find the onset time of rectal administration to be underwhelming. However, if you're not after a huge rush, rectal administration is a great way to go. It is relatively safe compared to ROA's like injection and it is well-absorbed.
The process is taking an oral syringe that you can get for free from any pharmacy, making your "shot" in a nearly identical way as you would if injecting. I generally use 5ml of water and it has never done me wrong. Make sure it is all solute and then lay down either on your stomach or your side, lubricate the tip of the syringe with either saliva or what have you, insert the barrel ~1 inch into your rectum, depress the plunger and then slowly remove (the suction effect can cause some of your solution to come out with the plunger if you move to quickly). You then lay on your side for 5-10 minutes at most then you should be good to go about your day.
I know everyone has their methods, but I have to say I've never had any issue with going into a bathroom, administering a drug rectally and then immediately walking back out. My point is, this is a pretty facile technique. It's hard to get it wrong. Let me know if you have any questions!