Its as easy as
Abduct prominant citizenry for bizarre experimentation, testing, and zoo exhibits on the home world.
Return those citizens with weak minds, and continuously transmit paranoid thoughts to their minds which can only be blocked by tinfoil. (It keeps them from realizing what really happened to them and warning everyone, because people think they are insane.)
Clone invasion army from cells collected during experimentation.
Wage war against the puny civilization you've corrupted.
Kids, Don't try this at home.
Hire wife's nephew as overlord to the enslaved citizens (He's not the brightest bulb in the batch, but he's loyal and it gets the Mrs. off my back) and become TIME Magazine's Alien of the Year.