The Manual Way
Most cross-bows will possess a part called a stirrup. It is a semi-triangular hole at the front side of the crossbow. You put one foot inside it while your leg weighs it down and then your body weight allows you to pull the string so it can be successfully cocked. If you prefer doing it this way be sure that your thumbs are as near to the stock as they can be otherwise it will not be done properly.
The accuracy will go down and the string might get damaged. This is one of the hardest ways to do it, because, as mentioned before, it will basically mean deadlifting 150 pounds or more, depending on which mode you have decided to buy. There are two better options that make it easier.