Summarize this article:

Compared to other yo yo tricks, the sleeper is fairly simple. Most basic yo yos of a reasonable level of quality should be able to perform a sleeper without any problems. However, some cheap "toy" yo yos which are poorly constructed may be almost impossible to perform a sleeper with. If you have one of these types of yo yos, consider upgrading to a higher-quality model to make it much easier to do sleepers and any other tricks that you may want to try. While certain high-end yo yo models can be expensive, most serviceable ordinary yo yos won't cost you more than $10-$20. For a stronger spin, consider buying a model with metal construction or added bearings — the extra weight gives the yo yo more momentum as it spins, making longer sleepers possible. To do a sleeper, you'll begin exactly as you would for a gravity throw. Loop the yo yo's string loosely around the middle finger of your dominant hand. Hold it upright in your palm so that the skinny end is sitting against the flesh of your hand. Curl your fingers around it slightly to support it. Hold the yo yo out in front of you with your elbow bent and at your side.

Summary:
Grab a decent-quality yo yo. Hold the yo yo in your hand palm-up.