|This is one of the two missions to setup the The Jewel Store Job mission. It is only available if you choose the Smart option at the end of Casing the Jewel Store. As soon as you finish the second of the two setup missions, the Jewel Store Job mission will automatically begin.|
You need to steal some knockout gas from a van that will appear driving around the map. You can either steal the van or blow open its back doors to steal the gas. Of course, stealing the van is a lot easier and won't draw too much attention to yourself, so make your way to where the van is location. Park your vehicle in front of the van, get out and then hijack it.
This will likely draw the attention of the cops, but it will only be one star, instead of possibly more if you use brute force. Lose the cops and then bring the van back to Lester's place to finish the mission.