A few questions

For first question you need export Doom 1 sky texture somewhere else and include it in pwad renamed "RSKY1" and put into TEXTUREx lump. That's how I replaced sky.

10 minutes ago, MysteriousHaruko said:

For first question you need export Doom 1 sky texture somewhere else and include it in pwad renamed "RSKY1" and put into TEXTUREx lump. That's how I replaced sky.

Okay, thanks Haruko. Ill have to figure out how to do that for my next set of levels.

For the third question. In vanilla style, at least for level names and int. screen text, Dehacked is used. I don't know how to do that. But in ZDoom style, MAPINFO is the way to go for adding level names, defining the images for intermission and ending and stuff like that.

For First question:

You can edit the sky you want to have in your level by MAPINFO.

For example:

SKY1 = "SKY2"

For Second Question:

Unless you want to put a custom Image in the intermission screen, there's no way to my knowledge to put Custom words in the intermission screen.

For Third Question:

If you are asking how to add your custom name into the intermission screen its easy but once again its done by MAPINFO.

For example:

//Map07 Being Dead Simple.

Map MAP07 "Your custom name"

As the other guys have said, if you're happy to target ZDoom / GZDoom, then you can do pretty much all that using the MAPINFO lumps. Have a read of this page for everything you need to know.