In one corner of my shelf of shame, there used to be several sprues of Ork Arrer Boyz from a Warhammer Fantasy Battles boxed set. They were older models, with less detail than Games Workshop's current work and they were fixed in a fairly limited number of positions. On their own, they were actually kind of ugly, but they were a great start for 40K conversions.
Before I could build them up, I had to cut them down. I started with the quiver, a huge lump of plastic molded to the backside of every model. They didn't really fit in a science fiction army (or at least they didn't fit in my science fiction army). But they were so large that a little care was necessary to remove them safely without damaging the models or my fingers.
I started by grinding off most of the plastic with a benchtop belt sander. You could probably do the same thing with a Dremel and a grinding bit, but it would take steadier hands than mine. Instead of holding one the tool in one hand and the model in the other, I could hold the model with both hands while the belt sander held itself steady. That made it much easier to deal with the models' odd angles. Once most of the quiver was gone, I switched to a hobby knife to clean up the details.
with enough time and care, the new surface could be nearly perfect. But I'd rather cover up a few errors with bitz from the Ork sprues. Here are a few examples from my previous unit of Slugga Boyz.
Once that's done, it was just a matter of trimming off the arms (or hands) with a hobby knife and replacing them with 40K accessories. Thank you, Games Workshop, for including a full set of Slugga and Shoota arms in every box of Ork boyz! But if you don't have the correct arms laying around, you can always trade for more or find extras on ebay.
Unlike normal 40K Ork models, the Arrer Boyz all lean pretty heavily to one side or the other. that leads to some pretty dynamic poses, especially if you mix and match the existing arms and the new ones. Here are a few of my favorite conversions from this unit of Shoota Boyz: