Plugging is simple, first get a hold of an oral syringe. You can get one from your local drug store either for free or for about a dollar, just say you need to give medication to your baby or dog. Now just add about 3-5ml of water to your drugs (you can use a spoon or bottle cap to do this), wait for it to dissolve, suck it up, and you know the rest. Make sure it goes in at least a couple inches deep and a bit of petroleum jelly makes the process easier.
There's not really an excuse to use anything but an oral syringe but if you must, you can break off the tip of an IV rig and use that. Make sure you break it clean and there's nothing jagged. Problem with IV rigs is they typically hold no more than 1ml, sometimes half that so you're gonna have to repeat the process a few times. If you're an H addict chances are you're constipated so you may not get optimal results, even then as an IV user you're likely gonna be disappointed.
Rectal bioavailability for heroin is still quite a bit lower than IV and it can take about 10 minutes to really get going IME. If you plan on using both meth and H together, which I can't in good name reccommend, try plugging a small dose meth first... this should induce a bowel movement so that anything you plug after will be fully absorbed.