So, what do you need to make a marking knife yourself? Chances are you already have what you need and if you don't the parts are really cheap.
The blade part is just as easy. I am using a 7/16" spade bit. If you don't have an old one laying around, the next time you go to the store buy a $3 blade. The HSS (High Speed Steel) blades are cheap and can be sharpened wicked sharp.
Because the shaft of the spade bit is almost the size of the end of the chisel handle it needs to be trimmed. I just cut the socket part of the handle off flush. The only other thing that needs to be done is to drill a hole in the end of the handle to receive the shaft. I drilled the hole just a little big so that the epoxy has a place to go. To drill the hole use a square as a guide or use a drill press. The hole needs to be as close as possible to straight, or it will look weird. I used a cordless drill and a square to keep mine straight.
First, mark the bit with a line were it needs to be cut. I cut mine just below the hole in the blade for obvious reasons. Cutting the blade can be done with a angle grinder or it can be ground down with a dry grinder. I like using an angle grinder with a cutting wheel because the steel really doesn't get hot and it cuts really fast. Using a normal dry grinder will get you there, but you will have to dip the blade in water over and over to prevent the steel from burning up. Once the angle is cut, you need to grind down the two sides so that they are as flat as possible. I also make the blade just a little longer while I am flattening the sides so that it is easier to sharpen. The two sides don't have to be parallel, unless you want them to be. I also suggest you do as much work on the grinder as possible . Once you leave the grinder we will move to the sharpening stones and we don't want to spend hours on the stones.
On the stones the first order of bussiness is to smooth out the two sides before you even start sharpening the blade. Only the one side really needs to be soothed out, but I like my tools to look nice too. Once the two sides are all smoothed out the rest is typical sharpening. I sharpen my blades up to 8000 grit stones. Either with water or diamond stones and then I strop. I strop and strop till I have a supper shinny and razor sharp edge. It may sound like the sharpening process takes a long time, but it really should only take 5 min. with a brand new unsharpened edge like this one. If it takes longer than that you didn't spend enough time on the grinder setting yourself up for success on the stones. In the future the sharpening will take about one minute to keep the edge nice.
Overall, this entire process took me just about 45 min. and I got a nice marking knife from stuff I had laying around the shop. I might make a few more or I might not. There is something nice about making your own tools. I also think that the process helps you with your ability to sharpen the tools you already have and teaches you how to alter tools that you have to make them better for your particular uses.