I'm not the best person to ask because I have never done it successfully. I have tried, I'm guessing constipation was the issue, even though I go every day.
Anyways, what you want to do is dissolve your bupe in water (the less you can use, the better - less possibility of leakage). Then, you can administer this anally with a needleless syringe (in the US, the pharmacy (chemist) will give them away if you tell them you are dosing a child - it's also called an oral syringe more commonly here). The best way to do this is to be laying on your side, and use some lube, and slowly insert the syringe, then push the plunger slowly until the solution is released, and after that I usually leave the syringe in place a moment or two before I attempt to remove it, once again to prevent leakage. The first few times, at least, you'll probably want to keep laying down for 15-20 minutes to make sure it has absorbed.
I have tried this three times. The first two, I think my dose was so high that I wouldn't notice effects anyways. The last time, I still didn't have good effects, so I'm thinking that I did not get good absorption. If you have fecal matter up there, it can interfere with absorption, making this a little tricky of an ROA for us maintenance users.
If you do an enema, you can eliminate that problem, you'd have to look that up on your own though, I just gave up before I tried that.
I have heard that plugging is the best bang for your buck, if you will (not sure if you use that expression across the pond), with better euphoria and effects in general short of IV. However, after my failed attempts, I ended up putting my time and resouces into making my nasal spray instead. Maybe it's just because I have a little bit of a sniffing fixation, but I find it the best way for me. It probably is not quite as good as plugging done correctly, but the thing I really like about my spray is that it has to be the hands down fastest way to administer. No fixing shots, or waiting for sublingual to dissolve, or waiting for solution to absorb... Just spray and go.
A little off topic there... Back to plugging, if you have any other questions, I can try to answer them, or I know there are some plugging threads here, I may even have one bookmarked, if so, I'll edit it in here after.
Here are a few links I found:
http://www.bluelight.org/vb/threads/395185-suboxone-plugged-(rectal-buprenorphine)
For plugging bupe specifically
http://www.bluelight.org/vb/threads/206901-Official-Rectal-Admin-Plugging-Thread
For plugging in general
(Sorry Evey, I don't know how to embed clickable links on my phone... Anyone know?)