You're a high ranking Yeerk. The Council of Thirteen has discovered a new planet: Earth. Your task is to plan the invasion of Earth. You have superior technological prowess and can overwhelm the inhabitants militarily, but in doing so will kill many of the population and render their planets' resources unusable. You can take over in stealth, as Yeerks are wont to do, but it will take some time, and you may not want to commit too many high-ranking Yeerks to the planet for that long. You can also try to appeal to the masses, by creating a cult or religion worshiping the Yeerks. Whatever the case is, what is your plan?
I have 5 plans to submit.
Plan 1
Start by taking over North Korea. Use military might to take Kim Jong Un and his top officers and advisors as Controllers. Then, leave. Appear as if the North Korean military has defeated you. Use their Cult of Personality to take control of the masses. Appear to the rest of the world like he is a little less trigger-happy, and that the civil rights in his country are improving.
Meanwhile, rely on the rest of the world to trust that you have defeated an alien invasion, which means one or both of the following: 1) the North Korean military is very powerful, and/or 2) the aliens are weak. This makes the people of Earth think to fear North Korea, but not fear the aliens, so no strategies will be built against the aliens. Meanwhile, as North Korea is getting more friendly, nations are willing to send envoys and ambassadors, even political leaders.
Well, the Yeerks that were "defeated" have stayed, either in cloaked ships in orbit or under the guise of holograms on Earth. These Yeerks can quickly take over anyone that comes to North Korea. Much like a plague in Plague Inc., where North Korea is solid red, and the rest of the world starts to be infested. As governments are taken over by the Yeerks, it becomes easier to take over the population.
On the other hand, if the rest of the world does not trust North Korea, then use the country as a base to start a military campaign. Use German WW2 tactics with Yeerk technology to quickly take over countries. Start off with smaller countries, and ask for forgiveness after taking them over. Once the rest of the world finally decides it's time to go to war, use cloaked Yeerk ships to take out military sites, especially nuclear launch sites and aircraft bases. Spread like wildfire. Like a zombie apocalypse, every human that is controlled not only takes one enemy away, but adds one more to your ranks. Spread like a Zerg infestation, with Yeerk Pools taking the place of Creep Tumors or Creep Colonies, and Creep being easy access to a Yeerk Pool. You might have some destroyed land, but most of Earth's resources should remain intact.
This counters the Animorphs in the original Yeerk plan because people would be a lot more likely to report strange things to the government.
Plan 2
This plan is similar to the first plan in that you spread from location to location, but it starts in a different place. This time, we are not starting as the leader of a country. This time, we are starting by taking over a small town, say a town of around 300 people. Once again, we start with military might, and take the entire town at once. This way, you don't have to worry TOO much about hiding schedules, behaviors, and infrastructure.
Now that you have an entire town, you start enslaving families at a time. People in your town have friends and family in other places. When you go over for a house party, bring a jar of Yeerks with you. (Leave them in the car). Drug everyone where you are going, and then bring in the Yeerks. Taking over families at a time instead of onesie-twosies is not only more efficient, but now that family can talk freely when they're at home. No situations like the Chapmans and Melissa, where the parents have to act robotic and she worries about her parents. Nope. Just a whole family of Controllers. Then that family goes to a house party, and the cycle continues.
Once a town has enough people, build a Yeerk Pool in secret there, so they don't have to commute to get their Kadrona rays, and the takeover can be more efficient. Once you hit critical mass in a town, take over the rest of the town by force overnight, and move on to the next one.
Where the previous plan used Blitzkrieg tactics or a top-down approach, this plan uses more subterfuge. You wouldn't even need to worry about world leaders. Once you have a critical mass of people, the world should be easy to conquer with few casualties.
This counters the original Animorphs in that if an Andalite crashed, there would likely be no Controllers living near the crash site, and thus no reason for the Animorphs to fight, or the entire town would be Controllers, and there would be nobody to turn into Animorphs.
Plan 3
Don't keep controllers on Earth. Simply do alien abductions to take people off the planet in droves. Start off with a conservative amount, say 100 people a night for a few weeks. As you get more Controllers, you can build more infrastructure and use more resources on the alien abductions. You should have an exponential boom in how many humans can be taken each night.
Start with 100 a day for a few weeks, and then jump to 250 a day, and jump again to 400 a day. In a matter of 2 months, you should have a force of around 13,000 people. Remember, we're building infrastructure at this point. After 400 comes 500, and then 750, 1000, 1250, and 1650. After 6 months, there should be around 125,000 controllers.
By this point, you should have enough personnel to separate into separate divisions. You have those in charge of building more infrastructure to house additional controllers, and those in charge of continuing the alien abductions to bring more and more humans in droves. Earth will know something is up at this point, but be powerless to stop the Yeerks. They wouldn't be able to track cloaked ships, nor would they be prepared to fight back.
As the amount of human Controllers grows, the system runs more efficient, and more people can be taken control of every night. Now you see exponential growth, a quick jump up to 10,000 a night for a week, and then jumping over and over again in a scale of 1-2.5-5-10 repeat (i.e. 25k, 50k, 100k, 250k, etc). Every week, you control more and more people. Within another 2.5 months, you have over 300 million Controllers and are taking another 50 million every night. Most are Chinese and Indian, but a lot are Russian and American as well.
Once again you hit critical mass. But this time, instead of critical mass to separate into infrastructure and abduction divisions, you move back to Earth and take over countries at a time. Every country on Earth may have lost 5-10% of its population to the Yeerks, but now its time to take over the entire country at once. Start with the smaller countries. With hundreds of millions of controllers, it should be easy to take over a country or even several countries at once. You have thousands of leaders planning invasions. Dozens work on each country, so that while one invasion is happening, the next several are already being planned.
This counters the original Animorphs by not basing the Yeerks on Earth, and hitting random locations around the globe, so the Animorphs don't know where to go in order to fight.
Plan 4
Large-scale sleeping gas attacks. Like Plan 2, hit whole towns at a time, but this time do so one after the other.
Counters the original Animorphs the same way Plan 2 does.
Plan 5
Find a forum dedicated to Animorphs discussion and make a thread asking for invasion strategies.