Here is a detailed tutorial about how to convert .gci, .gcs, or .sav saves into .raw memcards:

1: Install Dolphin emulator from [ Register or Signin to view external links. ]
2: Open Dolphin.exe.
3: Under "Tools", click "Memcard Manager (GC)".
4: Select the left "Browse" button.
5: Select a blank .raw memory card file. (If you don't have one, there is one included in the attachment.)
6: Select "<- Import GCI".
7: Select your .gci, .gcs, or .sav save file.
8: Close the "Memcard Manager" window.

If you want to use your new .raw file with Nintendont, then follow these steps:
1: Rename your .raw file to "<4_letter_game_id>.raw". (Example: "blankcard.raw" with Lugi's mansion should be named "GLME.raw".)
2: Place this .raw file in "<Drive_with_GCN_games>\saves\<4_letter_game_id>.raw".
NOTE: Make sure you place the .raw file on the same drive as your Gamecube ISOs. This is not necessarily the same drive that Nintendo is running from.
3: Set "Memcard Emulation" to "On" and "Memcard Multi" to "On".
NOTE: The settings in USB loaders may be different.

Attached are:
- A blank 8MB .raw Gamecube Memory Card: "blankcard.raw"
- A .gci save from Luigi's Mansion that I know works with this process: "GLME.gci" (In the save game, everything is completed except the final battle with King Boo.)
- A .wmv video of me demonstrating the process: "gci2raw_demo.wmv"

NOTE: The attached .rar file is actually in .7z format. (GBAtemp won't allow me to upload 7z files. ) If you don't have 7zip, then you can get it [ Register or Signin to view external links. ]

*All credit goes to Mr. Mysterio for the tutorial.*