While not as common as you might think, alien invasions are still a staple of science fiction. One of the reasons they are not done often is that they are really hard to do right.
In large part this is because if the aliens just wanted to kill off the human race it would not be much of a story. All that would happen is the aliens enter the sol system, then toss a few large rocks at earth. Even if we (somehow) manage to divert one or two, it would be very easy for them to toss more rocks down the gravity well and very costly for us to divert them. We would be hit eventually, and one hit with a rock big enough could render the earth uninhabitable for centuries.
Before I go on I will say that one thing they would NOT be here for is raw materials. If you want metals, water or anything else you would find in the earth’s crust it would be easier to get them from the asteroid belt or from a comet. There are a few things that you will only find in large planets (like uranium) but Mars would be a much better place to get them.
And as stated, if you want to kill all of us humans off all you have to do is drop a few asteroids.
So you want to have an invasion anyway, what do you do? As I see it there is one good practical reason; the aliens want to keep our biosphere functional. They would have to lack the ability to make an effective bio-weapon against us of course. There are any number of good reasons they would lack that ability.
If they want to keep our biosphere functional, the aliens would probably want to move in. Note that this would only apply if the aliens were short on time. If they could wait long enough they could just kill us off with asteroid drops then wait a few hundred years for things to clear off.
Say our aliens arrived and had to be on earth in less then thirty years. They are not really the sharing type so they plan to kill us all, and won’t drop rocks because they like the earth as is. And let us also say they got in without being spotted (hard but maybe not impossible). Their technology level is about fifty years ahead of ours.
The first thing they might do, beyond studying us, is to crash our economy. There is something like $1.2 trillion US dollars in circulation. What would happen if the aliens arranged for $10 trillion to find it’s way into the hands of a large number of random people? Ideally people would just start running around spending. If the counterfeits were good enough their would be no way for anyone not at the treasury to tell if they were fake.
After this I see a few options. First you could do a large number of assassinations. You would want enough that you would start to break the chain of command. In the US what would mean the President, VP, Cabinet, and all of both houses. You could do the same with other countries, the goal would be part confusion, part slowing down any reaction to further moves.
The first step of the invasion itself would be to take out all of our satellites as well as any launch facilities. You could then put your own satellite network in place, ideally with some Project Star Wars type lasers to stop ICBM’s.
After that you have the choice of either creating a beachhead and expanding from there or staying unreachable in orbit and taking out one city at a time until the human race can no longer field a military. At this point so long as you keep a presence in space large to recover from if everything on earth is lost you could not lose. The only way to lose would be for humans to get to space which would be between very very hard to impossible.