If a tool isn't too worn, you can follow the factory angle of the edge. Fix the tool in a vice. Yes, all gardeners should have a vice. The old fashioned way is to use a file to sharpen the tool blade. You can also use a handheld grinder.
Grinders are quicker, noisier and easy to get carried away with.
You can over sharpen a tool. Which is almost as bad as a dull tool. If you want to really hone up your digging tools, spray the blades with graphite which protects the metal. And it lets the tool slip like a butter knife into the soil. My shovels don't get this kind of treatment.