You will start this task from the O'Neil Brothers' Farmhouse and take down all the other meth dealers so that you are the only team doing this job.
Taking down the meth dealers will take up most of your time, be smart and quick (which you are as this is GTA!) Your aim is to get the Tanker full of meth to Chef at the gas station.
After you are done with the dealers, contractor would want you to get hold of the Tanker. Now here is a tricky part, this Tanker has HP, so its becomes the matter of utmost importance that you protect it in order to successfully complete the task.
Now find a truck and attach it to the Tanker to be able to move it. Since you will be under an attack from the O'Neil Brothers, be sure that your team is well prepared to take the damage and successfully deliver the tanker to the nearest gas station. When you deliver the meth tanker, your task will be done.