How do you make a knife at home with a grinder?

You can grind metal into the shape of a knife with a grinder, but it won't be good for much. Knife making is a skill that has to be learned. Shaping is only one part. The type of metal and hardness are also important. You will have to temper (harden) the knife after it is shaped. For this you will need an oven and quenching material. Not something you learn on Yahoo. Sorry.