First I'd tie the person up in a chair so they can't move. Then I'd like to somehow kill tissue within a very localized area. Like make it so just the upper shoulder dies, or just one of the pecs. Not sure what type of toxin would be needed for this or how to apply it but i would find out. Perhaps some type of bacteria or virus that spreads through the body very very slowly killing tissue along the way would work. Anyway, once they have a dead section, such as one of the shoulders, I'd inject it with worm eggs. The worms would be injected sufficiently deep that they took at least several days to get to the surface. At this point they would have grown a bit and the victim would get to watch worms slowly burrow out of the surface of their shoulder and wriggle around in it creating increasingly large holes as they slowly ate away at the person over a course of days or weeks. Since the tissue would be dead this might not hurt much, but it would be a nice mind fuck. Maggots could work too.
Then again maybe it would be better if I found some worms that would eat living tissue so the person could feel them wriggle around. Perhaps if I had the person drink nothing but water severely contaminated with guinea worms every day. But it would take a year for them to start emerging and really hurting. Wait, wait. If I'm reinfecting the person every day. Then that means that once a year passes, new worms will start emerging every day, in theory. And they can take a month or so to emerge, and each one emerging feels like a stab wound/fire, so if the body was just stuffed full the things, hmm.
Well either one suck I suppose. Maybe both at the same time would be good. The guinea worms will mostly be coming out of the legs so I could I do other kinds of worms via the first method in the upper body. Maybe use like, a different kind of worm on each body part, get a worm zoo going. And have worms just start erupting from every part of the person's body at some point, that would be fun.
Oh! Oh! And those worms I saw in House that swim in your eyes so that you see squiggly lines everywhere. Wait those worms were fatal. I hope there's a non-fatal worm that does that. I want the person to be looking at worms coming out of their entire body, with squiggly lines obscuring the view from the eye worms. Ideally the person should have a debilitating fear of worms.