What I've heard is the following:
1) Place "dstwoupdateb.dat" in the root of the MicroSD card, as well as the "_dstwo" and "_dstwoplug" folders.
2) Insert the MicroSD card into the DSTWO, then the DSTWO card into the DSi/3DS system.
3) Load the DSTWO as you normally would.
4) You should see the message "An error has occurred......." DO NOT TURN OFF THE SYSTEM!!!!
5) Instead, leave the DSi/3DS at this screen for 10-15 minutes with the DSTWO card still in the console. If you want to play it safe, leave it alone for 30 minutes.
6) After the 10-15 or (30 minutes), turn off the console and remove the flashcard.
7) Insert the MicroSD card into you computer and delete the "dstwoupdateb.dat" file.
***If you don't, then the DSi/3DS console will try to flash the firmware update the next time you get to the error screen. If you turn the console off or remove the flashcard before 10-15 minutes, your flashcard will most likely become bricked. Then you will have to find a DS Phat or DS Lite and another flashcard to restore the DSTWO***
8) Hopefully, when you load the flashcard again, it should run.