So I had to create some butterfly joints, which is something I hadn't done for a while and I know they can be tricky. The one thing you definitely need if you're going to create a strong yet attractive butterfly key is a set of seriously sharp chisels. I happen to have a piece of granite in my workshop which is perfectly flat so I use that (see image).
In essence, you flatten the back of the chisel first or it just wont give you great results. Then setting the angle at 25º (or 30º if you prefer a chisel you don't need to sharpen as often), you work down the grades of sandpaper as follows: 100, 220, 320, 600, 1200, 1500 then 2000.
You can trim your arm hairs at that point hence the name (the scary sharp method)... Some guys like to go even further but truthfully, after 1000 grit, all you're really doing in my opinion is polishing the chisel rather than sharpening it so what's the point.
With a larger chisel soon after this image was taken I actually started off with 60 grit and 80 grit to flatten the back because it was way off and taking forever on 100 grit.
A good tip for those that don't already know is to use a black sharpie to mark the part of the chisel you're sharpening to see exactly where the sandpaper is cutting, this way you can make alterations to the position of cut and the sandpaper you're using... Happy sharpening.