From the Australian site Krafty Farm Assistant comes this great little tool tip for drill bit sharpening.
Get two hex headed bolts and nuts and mount them in a piece of off-cut steel or timber so that two of their edges are touching (as shown).
Bolts of a suitable construction can be welded together to ensure they don’t move.
Sharpen your drill bit carefully using a bench grinder being extra careful not to be too heavy handed.
After only a few moments of grinding, remove the drill bit and test against the two hex headed bolts as shown in the image.
After touching up each cutting side of the drill your drill should fit squarely in the angle of the two bolt heads.
The accuracy obviously relies on how close you placed the two bolts together but with this guide it removes the need to eye ball the angle and hope that it’s right.
As someone points out in the comments, you also need to ensure that the bit being sharpened remains perpendicular to the shanks of the bolts to make sure the cutting edges of the bit are properly angled.
This tip was found on the awesome HandmadeTools site, an encyclopedia of tools that you can make yourself. It’s great to have a central tool-making project repository like this. Many of the tool project articles they list are found on various specialty project forums where content can often be hard to locate. HandmadeTools also offers a free ebook of their most popular tool-building projects when you sign up for their mailing list.