nesdev.org

how do people extract an individual game from a multicart rom?

With a hex editor

Of course, this where the corrupt SMB1 dump came from.

how long does that take? is it hard?

It's not hard if you know the bankswitching of the muticart, can use a hex editor and in some cases hack initialization code.

I knew that. 8) "Tropical fishing is what i call it"!

Use HEX editor, for example:
You want rip STAR FORCE game:
- download Star Force rom file from any rom site
- open HEX editor and see first bytes of Star Force PRG rom data and CHR data
- open Dumped multicart
- use SEARCH option in HEX editor and search Star Force PRG and CHR data first bytes
- Mark/Cut/Paste Data into new file (Join PRG+CHR)
- add header

any other games in multicart - similar method

Or if you have only the multicart, and it runs in an emulator, you can replace the first two steps:
- start game in emulator
- open PRG/CHR memory viewer and see first bytes of Star Force PRG rom data and CHR data
Or:
- set the emulator to log mapper writes
- start game in emulator
- from the log, determine which banks are being used

how do you add a header?

With a hex editor

This thread reminds me of that one time I extracted the SMB rom from the 3-in-1 Nintendo made (SMB/Duck Hunt/World Class Track Meet).

And the time I went the other way.