Welcome to the ancient and secret Order of the Sith. As the latest in a long line of apprentices and masters, it will be your task to take over and then rule this galaxy through sheer force of evil. And the Sith can tell you exactly how best to be evil.
As a certified evil person, you are free to ignore the constraints of morality and do whatever helps you regardless of what other people think. Don’t do that. Instead, make sure to always go out of your way to be evil, especially when it hurts you. How else will people know you were serious about being evil? One of your generals was foiled by the protagonists? Kill him. This will encourage his replacement to avoid doing anything ever, just in case it fails. Your apprentice got defeated? Have someone kill him and recruit that guy. Your new apprentice will know that means you’re perfectly trustworthy. You have ridiculous amounts of energy available? Put it into a planet-destroyer. Science has shown that this is the best possible use of 10^32 Joules.
Murdering billions of people simultaneously is a well-known way to make the survivors hate you. Don’t worry about that; you are completely invincible and can feel free to kill billions whenever your child misbehaves.
If your unstoppable superweapon gets stopped, you must learn from your mistake. Build another one. And of course you should include the same alleged “weakness;” you know it’s unstoppable so it’s not like there’s any extra risk. Besides, it even lets you know their battle plan in advance.
Many evil overlords recommend maintaining a public image masquerading as someone who isn’t puppy-murdering evil. They’re wrong. Straight-up informing people not to trust you can’t possibly go wrong. Then you can feel justified in doing exactly what you said you would whenever you feel like Altering The Deal.
On that note, make sure to use a suitably terrifying appellation. Every name creates a strong first impression. You want one that says not only “please don’t laugh” but also “I am one hundred percent super trustworthy definitely.” This is why all names should be obviously evil. It should be something evil-sounding, like “Darth Nefarious.” If your name is Darth Fluffypants, nobody will be quaking in fear, and it’s obviously either quaking or laughing. There is no middle ground.
The most important thing to remember is that there are always two Sith. One master and one apprentice. Since the Sith are in possession of hoards of ancient lore, they must make sure to pass it down safely so that none of the knowledge or skill gets lost. The master teaches the apprentice everything he can before the apprentice gets around to murdering him. This guarantees that no information gets lost between generations, and also that they can always trust one another.
Since you know you and your apprentice can trust one another, feel free to do everything in your power to make him want to betray you. Torturing his long-lost son in front of his eyes might be a good start. Make sure your monologue includes talking about how you don’t need your current apprentice anymore. Since no apprentice Sith has ever turned on his master before, this is completely risk-free.
On the subject of turning on former masters, always remember that you are unbeatable. If someone who you know is more experienced tells you that you can’t win because he has the high ground, he’s wrong. Period. Like Henry Ford says, if you think you can do it you’re right, and if you think you can’t do it you’re still right. So you have to always believe that you will win, and then you’ll never wind up dismembered and on fire and left for dead by the good guys.
While trying to take over the galaxy, you need to be where all the powerful people are. If there are people known to be able to see the future or sense what people are thinking, go live next to their headquarters. They probably won’t notice anything.
Finally, you must always gloat. If you have an enemy at your mercy, make sure to gloat. Make sparks by slashing your lightsaber against the ground for no adequately explained reason. Even if he is uninjured and has superpowers, there is no possible way he could interfere.
With all these advantages you will gain from being Evil, you cannot fail to conquer the universe. By doing exactly what they did, you will succeed where the last few thousand failed!