1. Do your best with genetic engineering. Create a huge variety of very different viruses that lead to human infertility but no other detectable symptoms, and make it transmissible by air and sexually. Make them transmissible to mosquitos, pigeons, cattle, rats, bats, cats and dogs, to ensure massive virus circulation. Make sure that the viruses spreads and infects people quickly, efficiently and silently.

BONUS: make the viruses induce production of pheromones on infected people, so they are more attractive than non-infected people.

1. Spread the viruses everywhere, in every city, town or indian village.
   

2. It will take some years for the scientists to understand what is going on and to discover, study and analyze the viruses. Many more to propose efficient ways to prevent further infection or promote efficient and widespread vaccines. Further, you designed a lot of very different viruses, not just one, so it is very unlikely that people will be immune to all of them. And they will need to be discovered, studied, analyzed and vaccinated independently each one in its own way.

3. When people becomes aware of the existence of at least some of the viruses (likely something between 2 and 8 years after you started to release them), spread misinformation about them, including misinformation in scientific reports and articles. Spread the type of misinformation that makes people paranoic, skeptical and more likely to spread the virus, and that also slows down scientific investigation.

4. Try to kill any scientists that are publishing seriously breakthrough work and studying what is going on. You may shot them down, poison them, trample them in the street, whatever. Make it looks like an accident or that some crazy fundamentalist guy did that. You will be unable to locate some of those scientists and it is hard to know what everyone is researching, but each kill is a point for you.
   

Now, the fecundity drops worldwide, and fewer and fewer babies born each year. The scientists will start to work on it, and in-vitro fecunding will be a high profit business, but this is too few time (2 to 8 years) to efficiently address the issue or even to make everyone fully aware and non-skeptical, specially with widespread misinformation.

So, it is time to go to part 2 before the scientists make further progress.

Part 2: Destroy the economy and make war

People need food to live. With few food available, people will die from hungry, and kill from hungry too, starting wars, crime and violence.

1. Atomic bomb major cities everywhere, all about the same time.

2. Spread orange agent, radioactive elements, poisons and/or viruses in plantations, waters and forests all around the world. This would make safe food really scarce.
   

3. Make each nation blame another one for the bombings and the destruction of crops, spread misinformations, manipulate media. This will trigger WW3.
   

Now the industry, the production and economy will be seriously damaged everywhere. The world is at the greatest war in the history. A lot of people dies. Survivors are struggling in the ruins, without anything safe to eat or to drink, people will run away to the countryside destructing whatever left from the economy there. The fertility problem is regarded to a second-plane and no one will be able to continue research on it anymore. There is just anarchy, crime, war, violence and misery.

Further, the chaos and anarchy will strongly benefit the spread and circulation of the viruses by moving a lot of people and resources around. People will likely disrespect any safety measures previously adopted to avoid further contamination, since getting food and water is always more important than this.

Part 3: Game Over

In a few more years (lets say 15 years) at least 98% of the pre-existing population will be dead, and most of the survivors will be in bad health, miserable conditions and unable to have babies.

1. Now you just need to chase any possibly reproductive survivors. The easiest way to do that is to use drones to look for kids. When you find a kid, just shoot them (use your drones for that) and look for their parents somewhere around, and shoot them too. Or better, just shoot everyone alive, except if you guess that he/she might unintentionally show you where other survivors are hiding around, paying special attention for kids, then kill everybody. If you suspect that they are in a larger tribe or family, bomb the area for some days, search carefully using the drones and drop some radioactive dust in the area.

After a few more years, there would be no more than a few handful survivors around, probably less than 1000 in the entire world. They are too scattered, hidden and fearful to recolonize the planet, even if they have any mean to communicate. Even if a few kids survive and reach adulthood and can reproduce, they are still too few and too scattered to make the human race survive, so the humans will become extinct soon.

Mission accomplished. It took something like 20 to 30 years, but it is finally done.