Inspired by a Reddit prompt:
I have a feeling my answer would get censored. Feel free to give your own.
Evaluate Earth's space capabilities. Secretly colonize the Solar System where possible.
Prep asteroids and comets for Earth bombardment. One asteroid should contain a burrowing probe capable of taking control of the Earth's magnetosphere and disabling her radiation shielding. Weaponize the Sun to target Earth with solar flares, or appear to.
Abduct test subjects. Develop slow-incubating nonlethal virii that reduce IQ and testosterone of Eurasians, both current phenotype and descendants' genotype. Prep cyberwarfare package.
Release biological virii in all major ethnically Eurasian cities simultaneously. Wait until humans start noticing the virii. Release cyber-warfare payload onto the Internet. Wait until effective cyberwarfare countermeasures commence. Simulate a huge solar flare that EMP fries Earth's non-hardened electronics. Infected will flee the cities, spreading the virii. Wait until spread is checked or saturation achieved.
Deliver asteroids, obscuring Earth's skies. Eliminate remaining human space assets. Begin openly colonizing the Solar System except Earth.
Monitor radio chatter for a decade to gather intelligence on survivors. Then send in the drones to mop up with WMDs. (Environmental degradation is no big deal. Mars is clean.) Make honeypots out of destroyed shelters. Maybe drop toxic comets to poison the atmosphere.
Begin terraforming Earth. Establish navy and undersea colonies. Maintain air superiority. Build amphibious landing armies. Take continents, starting with Antarctica.
Collect gene samples of extinct native flora and fauna for archives and future zoos.
Subvert and usurp
Surveil and then infiltrate Earth.
Cause three generations of unprecedented prosperity in the USA, EU, China and Russia via technological advancement. Send infiltrator tech "inventors", CEOs, coders etc. Render the population degenerate through luxury. Maneuver unqualified leaders into power. Gain technological control over key systems such as stock markets and strategic defense. Encourage immigration and miscegenation to reduce IQ of Eurasian IQ 100+ ethnicities. Encourage IQ- and testosterone- reducing environmental and nutritional pollution and social policies. Develop suborned AI.
Cause rapid destabilization and economic collapse. Support simultaneous left-wing singularities in the USA, EU, Russia and China.
Ally with Democrats to counter the USA. In Russia, encourage dysgenics to reduce IQ. In China, exploit populist unrest and cults to cause civil war. Keep Europe a militarily-feeble satellite. Foment conflict between Russia-China and NATO. Precipitate a nuclear war. Infiltrate command and control and expend all WMDs, prioritizing IQ Eurasian IQ 100+ population centers.
Establish a world government and unified religion combining Judaism and Christianity, seated in Jerusalem. Restore prosperity. Subvert and destroy any holdouts. Register all humans. Require a digital implant to buy and sell.
Propagate IQ-reducing virii. Populate vaccinated elite with gengineered psycho-cybernetic thought-controlled humans perfectly loyal to aliens (cloaked in religious mythos).
Proceed with direct conquest plan, using enslaved humans as supporting partisans. Then sell the useful ones to alien colonists as chattel, and liquidate the remainder.