For starters, you nix 3+e style multiclassing. Thankfully, in 5e, that just means not using that option.
2. No Feats. Same thing. They're an option in 5e, so just don't use them.
3. Roll your stats. No array. No "point buy."
4. Preferably, no Drow PCs. If you are in love with Dragonborn or Tieflings and are just looking for a 1e "flavor", then sure, keep the dragonman and half-demon as PC options.
5. Limit Eldritch Knights and Arcane Tricksters to Elves, Half-elves, and gnomes. Gnome Eldritch Knights are still limited to Enchantments and Illusions (as Arcane Tricksters, for the 1e "Fighter/Illusionist" MC gnome character option).
6. Lethality. Full stop.
6a) No "full healing" overnight. No "short rest" HD rolling. If anything, allow a single HD roll for overnight.
6b) Encounters that exist as they make sense within to the world. No "balanced" encounters and definitely no expectation of auto-success.
6c) Traps & Hazards. They're a thing. All over the place, not just in nice little bundled packages/obvious places where the thief knows they are before walking in.
The 1e of me experience is not/was never about "DM vs. PC." It's still a communal story of heroic characters doing daring-do. But it's not "the PCs are entitled to..." ANYthing. The PC's should not and can not EXPECT they can handle everything they walk into. If you are not willing to run away in the face of defeat, imminent, assured, or otherwise, you aren't ready for 1e.
7. Alignment. It is also a thing. Monks must be Lawful. PALADINS must be Lawful (if not actually Lawful Good, which would be truly traditional). Barbarians, I suppose, should have to be Chaotic. Rangers must be Good. Druids must be [True] Neutral... Assassins, since I guess it isn't obvious anymore, must be Evil. Alignment matters -yes, moreso for clerics, druids, paladins moreso than anyone else. Don't like it? Don't play them- and prolonged or egregious deviation from your proposed alignment CAN (and WILL) result in the loss of your deity's connection and/or class-specific powers and abilities.
8. Auto double damage on a natural 20. Auto fumble on a 1.
9. Side Initiative. You all go first or they all go first. The other side, all, goes after and back and forth. Round by round if you like.
...ummmm...there's probably more. But nothing else is coming to me nad those ought to give you a good 1e "feeling" game.