Simply go onto the home page, enter the name of your company/title of your draw and in the second box, the number of people in it.
Press the submit button. The site will then create random pairs for your hat, and a blank name sheet for everyone to write their number on.
Tell each participant to write the top number on their draw slip on the sheet, they need to keep the bottom number secret.
At the end of the draw they will find out who they are buying a gift for, by reading the name against their lower number.
Even if the draw is done over a period of time, and there is just one slip left, everyone will know their top number, but not who they are buying for, so this works even if everyone is not present for the draw.
If you cutting out the draw slips and have a really good memory, you should run the generator 2 or 3 times. Put each set draw slips in a seperate identical pots, mix up the pots and choose one at random. Throw away the other slips (without peeking!)