I personally use Crispy Doom, I find it the most accurate in terms of playstyle and looks of the original doom compared to other source ports. It's one of the most popular and reliable ones. Other speedrunners use CNDoom or PrBoom, those are also popular. A more aesthetically pleasing port is GZDoom, but that isn't an option for submitting runs.

If you have doom already copy the iwad file from the install and paste into crispy then it should work.

I can show you how to record demos once you get your hands on a copy of doom. You can download it easily online buts its better to buy it in case the FBI comes knockin down your door

What do you mean?

The best way is to record demos with a windows batch file (which I can elaborate on if you need) and post it on doomworld.com. I used to use an app called GeForce Experience that captured a certain amount of time on your desktop to use as video evidence when submitting runs.

You can submit from "steam". Thing is, it runs in a DosBox, and it is by far one of the worst possible ways to play. (And DosBox is part of a list)

Yeah sure. I'm planning on making a video guide on recording demos today but if you are able to follow written instructions, this is how-

1.Create a windows batch file by right clicking on the desktop, hover mouse over new and create a new text document. Rename it however you want but it should end in .bat

2. Edit the batch file and add these parameters (if you are using crispy doom which is what I use)

This is what it would look like if you recorded an E1M1 demo.

-crispy-doom -skill 4 -warp 11 -record e1m1

If you choose to play on dosbox, the first parameter may be -DOSBox.exe but I'm not sure.

3. Save and drag the batch file into the source port folder along with the doom wad file.

4.To record the demo, click on the batch file or create a shortcut of the file on your desktop and it should start recording.

I suggest mapping a key to reset the demo when you mess up so you dont have to restart every time.

Every time you reset the run you just played will save and a new one will start recording.

All demos should save in the source port folder after the batch file stops operating.

Again, I'm planning on making a video guide to make it much clearer, but hopefully you get the idea.

https://www.speedrun.com/doom1/guide/wfq0i

Heres the video guide. Hope this helps!