When sharpening a chisel it is necessary to use a consistent angle to create a flat blade edge that will cut into the wood correctly when you are using it. Sharpening by hand makes it nearly impossible to achieve a truly flat surface since hand motions are incredibly difficult to repeat exactly and without variation. Building a chisel sharpening jig will allow you to sharpen your chisel by hand but still have a straight, true, flat blade edge to properly cut into wood. Chisels will have one 30-degree face and one 25-degree face that meet at the edge.
Measure a line 30 degrees up from the long edge on the side of your piece of lumber. Keep the line in the middle of the piece. Cut one eighth of an inch into the wood with the saw. Measure a 25-degree line on the opposite side of the piece and cut into it one eighth of an inch.
File out the cuts you've made until they are wide enough to hold your chisels. A file is usually about the same width as a chisel. Test fit your chisel into the jig.
Lay the wood on your whet stone so it lays flat against the stone and the chisel rides in the channel you just created so the appropriate face of the chisel is flat against the surface of the stone. Test the fit on both faces of the chisel to make sure you are using the correct angle with the correct face of the chisel.
Sharpen the chisel on the whet stone with the honing oil; use a push stroke only. After 10 strokes look at the edge of the blade under bright light. If you can see any light reflecting off the edge, then it's not completely sharp. A sharp edge will look like a black line when it's ready. Once it's sharp, take one stroke on the back to remove any burrs from the edge of the blade.
You can use this same jig with a bench grinder by clamping the piece of wood level with your grinding wheel and sliding the blade of the chisel farther up in the angled channel so the wood doesn't come into contact with the grinder. Slide the blade back and forth in the channel to make contact with the grinding wheel. Finish your sharpening on the whet stone.