It all depends on what you have.
The first thing you need is something to sharpen it against.
In modern times, we could use a grinder, a Whetstone, Oil Stone, Water Stone or Diamond Stone.
The basics are, hold the knife at a 17-20 degree angle, sharp bit against the stone and almost flat, then slide the knife along the block pushing down with your finger tips.
If possible, you start with a courser grit, and gradually go down the grits to something finer. This makes the blade extremely sharp.
If you would like to see a great guide about this, visit WikiHow – How to sharpen a knife.