I went on a friends fixie for the first time a few weeks ago and had never rid one before. Took about 3 attempts until I could skid. It's all technique based. Everyone has their own methods but I found that when my drive foot was almost as far down as the crank arm can go to the floor I would just push against it with force and this would skid. That's a pretty standard method as you can see in the video above. Just keep trying, although it does get tiring after a while.