I can't find a solid answer

If it's Doom.exe compatible (a couple of examples are Batman Doom or Aliens TC), then they'll come with instructions for running it.

I can't really go much further than that as they tend to have different instructions, sorry! I would highly suggest a limit-removing source port at least anyway, as a very large amount of WADs are boom compatibile at the very least (I would say that actually the majority are, but I don't have the statistics here). I believe Chocolate Doom can run the very few older WADs that have issues if you still can't get into them.

Short and solid answer for the perfectliy compatible with the vanilla engine PWAD would be: doom.exe -file yourpwad.wad
You do this either in the real DOS or inside the (par)virtual machine like DOSbox.
Long answer was given by Projectsonic just above.

P. S. Also Romero himself cosidered this article worthful: https://doomwiki.org/wiki/How_to_play_or_start_wads

So this is how everyone did it in 1993?

Generally speaking -- yes. More advanced launchers appeared later.

To avoid typing all and every command line switches each time for the often played WADs there are so called 'responce files'. You just write your switches one per line, save the file and then feed engines with it like this: doom.exe @yourpwad.rsp
Moreover, most of the people use this method even now. Because this is exactly what modern DOOM launchers do.

But there were own complications in the vanilla era: original engines were unable to load sounds and sprites form PWADs. Therefore installation step arised for some advanced 'mods' of that time. When you had to merge some modded resources into the main IWAD before playing. See deusf and deutex. Later came idea to hack the engine itself. DEHackEd patches had to be integrated into main doom.exe as well. So, running some complex setups involved use of the appropriately complex BAT files. Which had to handle backup of the main IWAD and the EXE, merge required changes into the IWAD and the EXE, run the game with custom responce file and upon exit unroll everything back. Optionally saved games had to be dealed as well.

Thus, 'perfectly compatible' PWAD of that era is one, that containes no modded resources but new levels and graphic lumps for the wall and sky textures.

--- Update ---

Forgot to mention. DOS verions of the DOOM games available at GOG are late releases. They come with DM -- the launcher primary intended for multiplayer modes. But it can run singleplayer sessions with custom PWADs as well. In case you are interested.

Thank you very much! That makes a whole lot more sense :D

One more tip. If you really intend to run some WADs wth the vanilla engine, it's better to use limit removing hacked version.
Namely: DOOM+. (Yes, the same homepage and same author as of PrBOOM+ source-port)
What is changed.
This hack on one hand makes original dom(2).exe much more tolerant to maps with more details; on the other hand game behavior and feature compatibility remain completely authentic.

So will this work with Sigil? Because the answer I got from the steam forums was no you absolutely gotta use gzdoom

Check this:

https://www.youtube.com/watch?v=dOJfikjIqWg

Well, GZDoom kiddies always insist on GZDoom, obviously. :/
Already answered about the Sigil in the Sigil topic. In short: you can't run Sigil on the unmodified vanilla engine. You need both DOOM+ hack to improve engine limits and DEH patch to prevent last level of the episode from ending prematurely. On the first glance modified this way doom.exe works fine with Sigil. But I didn't play it throughly myself and don't know if problems would arise in the middle of walkthrough. Still waiting for my box to arrive. ))) And, well, commercial soundtrack by Buckethead is out of question in this case, since vanila engine can not play MP3.
If you are not an old-timer it would be better to use a source-port. Chocolate DOOM and Crispy DOOM will give you about the same experience as vanilla DOOM in DosBox without extra setup steps. Those ports even render game graphics in low resoltion. Just use sigil_compat.wad with them. Demos bundled with WAD were recorded with PrBOOM+, threfore this port is fine too. Just add -complevel 3 to the command line for the proper behavior. That's BOOM they use. Old source-port build for DOS.

--- Update ---

Ok. Here is an archive which allows to run Sigil with GOG/Steam Ultimate DOOM for DOS without source-ports (and without MP3 music!!!). Just exract everything from this ZIP-file to the directory, where doom.exe of your Ultimate DOOM for DOS resides.
Run with sigl.bat. That batch will take care about moving and backing your saved games and run Sigil with pre-hacked engine. Pre-hacked engine is sigl.exe. Correctly renamed for DOS evironment sigil_comp.wad is bundled as well.

Just in case VirusTotal report on sigldos.zip.

Yes, I know. I am assuming the OP wants to go as close as the original game experience as possible, and maybe even being able to run on DOS on vintage hardware. Otherwise why bother with all this.

Thank you very much for all this!! :D