First, you need loyal henchmen. And by loyal, I mean fanatically loyal. To do this, you have to first adopt (so you can later co-opt) a cause. These become your holy warriors; men who will not question your orders, because you are one of them. Examples:
- Mua'dhib's Fremen Fedaykin in Dune
- Harry Potter's "Dumbledore's Army"
- Felisin's Army of the Whirlwind in the Malazan series
- Darth Vader's Ngoh assassins in the Timothy Zahn Star Wars novels.
To obtain such henchmen, you must set out with ostensibly altruistic motives. These are the people who must believe you are worth killing and dying for. Therefore, you must
be that person to them. You must adopt and actually advance their cause. This also means that you can't start out as a BBEG, or you attract no loyalty (note that this fact should significantly nuance BBEG's in your campaign).
Second, you need a base of operations. This is one aspect of traditional D&D that is woefully under-emphasized in modern editions: the taking and holding of a keep. The secret base of operations is also a place to inculcate and reinforce, through proximity, exposure, and peer pressure, your world domination philosophy to your henchmen. The goal of the keep is not just a projection of power, and a defense against enemies, but also a place to isolate followers and more strongly bind them into the cult of personality that a BBEG must foster if he wishes to rule the world.
Taking over your surrounding country-side will be a task and a half. Sovereigns will not cede their lands simply because you're nice to them. You must take them by force. However, paradoxically, you must have the sympathy of the people or surrounding countries will rally to your target. If you win, the subjugated people will hate you. Low-hanging fruit in this regard is any local kingdom who abuses its people. These are areas where you can infiltrate, provide humanitarian aid, destabilize the government, and hopefully wage war with the support of the populace.
Once you have amassed to yourself some land and people, you enter the realm of international politics. This will be the hardest part. Attempting to forcefully take over the world almost never works. Ever. BUT, creating a sphere of hegemony is easy if you continue acting in the interests of other countries, especially smaller ones. Providing incentive to smaller powers to conglomerate into a confederation is easy enough, but will require significant military power. The hiring of skilled generals and mercenaries will be required. Obtaining the actual sovereignty of these smaller nations is unnecessary; the cohesive whole will choose a premier you can control, as long as you maintain a healthy spy network and control over local party politics in each canton. Examples from history:
- The Aegean League
- The Roman Republic
- The Holy Roman Empire
- COMINTERN/Warsaw Pact
So, you've got yourself a little empire, even if it's not technically called that, and you have a core of fanatical followers. Your people love you because you have been actively seeking out the resources you need to enrich your citizen's lives. Here the non-BBEG can retire, and go down in history as one of the greatest figures the world has ever seen. But that's not what you want, right? Take over the world, you say?
Well, from here, you can't unless you go full-bore BBEG. We're talking about going the whole nine-yards: subverting religion, assassinating unfriendly foreign figures, mercilessly destroying your opposition, and invading and enslaving whenever you get the opportunity.
And since this all is going to take you a couple hundred years, you might as well get started on that lichdom ritual while you've still got the time.