Step one: Start pedaling at a medium pace
Step two: Lean forward to displace weight over the front wheel
Step three: Your strong foot needs to be at the back pedal, when the pedals are parallel with the ground
Step four: Apply a lot of pressure on your back foot
You should skid a bit, be careful not to hurt your wrists in the process, it can get tedious and a fair bit painful if you are doing it non stop. Practice until you are able to skid, and then try to work skids into your regular riding.
There are many different types of skids you can do, like the regular skid, through the frame skid, over the handlebars, and other creative and insane variations. Skidding helps you stop on a fixed gear bike, due to most fixies being completely brakeless.
It is easy to skid, so don't give up on yourself. If you can't do it after day one, try again the next day. It takes time to learn, so if you don't want to dedicate a few hours a day to practice, you should stick to a regular road or mountain bike.