Draw or steal a fantasy map; something that doesn't tell the players it's set in the so-and-so campaign world. Essentially, a stand-alone chunk of mostly-wilderness land big enough it takes you weeks to travel from one end to the other.
Then you plop down (i.e. choose) a "starting town" and the seven dungeons from Yawning. That one is described as a "citadel" or a "temple" doesn't mean you need to have one of those on the map, the dungeon could be sufficiently overgrown or "sunken" to explain why it's only marked as a set of standing stones or a cave entrance on the map.
Then you seed the valley with at a couple dozen semi-permanent "forces" like a tribe of goblins there, a dragon here. This can include "civilized" forces such as a force of Knights protecting a Monastery, or the militia of a city. Be sure to include a couple of presumptive higher-level "vendors" (a wizard's tower, the Church of Wisdom etc).
Now select your first dungeon. Could be Sunless Citadel, but maybe that's so low in level it's already known in town, so perhaps Forge of Fury. Who knows about the place? (Where to find it, essential clues to notable traps or treasure or monsters, and the like. And who knows about that guy? This way you'll sprinkle clues as breadcrumbs for the party to find. You don't need to do all dungeons right away, as long as they always have at least one clear lead to follow up on.
Also include "questgivers" that aren't related to the main dungeons. Could be as simple as "collect thirteen goblin ears for me and you shall be rewarded with this magical dagger".
Then random encounters and wandering monsters will do the rest, and you can find those on the net (there's even threads here on ENWorld asking for good hexcrawl resources). Don't forget about these - it's paramount there is risk and cost involved with travelling and especially outdoors resting.
And you're done. (Maybe you will want to tweak the 5e ruleset to a more old-school sensibility, but that will have to be the topic of another post)
Then, when (and if!) a certain monster or NPC catches the players' fancy, you can expand its role to start a more traditional story-line (something like "the princess you met? she's been kidnapped, and rumor says the Dragon lives in something called "the forge of fury"...)
Back in the day we didn't have APs.
This.