1) Obtain DevMenu.3ds (There is a link on Gateway's download page)
2) Place DevMenu.3ds onto the Red Card's microSD card.
3) Place BlueCardFix.cia (found in Gateway's download) onto the system's SD card.
4) Boot the system and use your exploit of choice. Hold L while doing so in order to reach the Gateway Menu.
5) Highlight "Boot Gateway Mode", and hold B. The text will change to "Boot Gateway Sysnand Mode". While holding B, press A. It will ask you to confirm you want to boot sysNand by pressing X. Do so.
6) Press Select to bring up the rom menu, select DevMenu, and run it.
7) Use DevMenu to install BlueCardFix.cia. This will be installed to sysNand, so you will be able to use the blue card.
Now that this is done, reboot the system. The Blue Card will work in sysNand, before you use any exploit to enter Gateway Mode. It will not function in Gateway Mode or in emuNand.