How to make custom messages and episodes?

Hello everyone! I am working on a megawad for Doom 2. So i have a question, how do i change the messages from the end of the episodes, and is there a way to make episodes in doom 2? Thanks for everybody who is going to help!

You can change the texts by creating a DEHACKED lump and editing the proper bex strings.

[STRINGS]
C1TEXT = "This is the message between MAP06 and MAP07\nAbout the infested starport and stuff"
C2TEXT = "This is the message between MAP11 and MAP12\nAbout allowing Earth's spaceships to flee"
C3TEXT = "This is the message between MAP20 and MAP21\nAbout the corrupted heart of the city"
C4TEXT = "This is the final message after beating MAP30\nAbout the icon of sin dying"
C5TEXT = "This is the message for when you're entering the secret level of MAP31"
C6TEXT = "This is the message for when you're entering the super-secret level of MAP32"

Type your message on one line, between double-quote marks. Use \n to make linebreaks, \n\n to make paragraph breaks.

If you need more flexibility than that, you'll have to use MAPINFO.

Use MAPINFO to define episodes starting at MAPxx where xx is the number.

Use clusters to determine where text screens appear and cluster definitions for the text.

Gez's solution is what you'll want to go for if you're aiming for something compatible with vanilla (or Chocolate Doom); Nevander's is what you want if you're aiming for something playable in GZDoom and many other MAPINFO-supporting source ports.

For vanilla you can't use BEX strings actually. Pure dehacked string replacement is more complex and also the patch has to be an external file rather than an embedded lump.