https://forums.dolphin-emu.org/Thread-updated-first-post-how-to-play-brawl-project-m-etc-unofficial said:
-Making the virtual SD card-
If you already have an SD card with all the necessary data, (The mod, Gecko OS, etc.) just insert that card in your computer, open WinImage and follow the steps below. If not, keep scrolling down to the text in bold.
1. On WinImage, click on "Disk" and select "Use Removable disk *the letter your SD card was assigned*". Then, still in this menu, click on "Read Disk" near the bottom.
If WinImage gets picky about not wanting to read the card, it's because you opened a folder of the card. So close WinImage, re-insert the card and try again without opening any folder such as my computer.
2. After doing the above step, a save window will pop up. Change the save type to "All files (*.*)" and save the file as "sd.raw". After WinImage is done saving, it will conveniently open the file for you.
You have now successfully backed-up your sd card and may close WinImage.
3. Now that the SD card is ready, place it in Dolphin's "Wii" folder