It's not hard to install it you just pt the CIA on the 3DS SD card the select it on the Dev Menu, the dev menu will install it itself and then it will be a gift like the games you download from the eshop.
About how tu use GW you should wait till the update is out as it most likely wont work the same as on 4.5
So, let me see if I understood well, pals:
For example, I want to install "altered beast 3D .cia". (when all the exploit is ready and working, whatever it is)
-I put the cia on 3DS SDcard.
-Open DevMenu (where should be DevMenu? it comes with the gateway? or must I search it and drop it on the microSD?)
-Install the cia.
-???
-Open present. Profit!!
But, I have some question. After instaling the cia (it install in the SD, right?) can I delete the .cia archive for space management purposes? I mean: DevMenu creates a new archive (installed game) from the .cia (making the .cia eliminable), or it relocate the .cia (making it not eliminable because the 3DS uses the cia itself)?
Also , about SD and microSD capacities... I'm not getting a 32gb microSD until 6th Jan (Three Wise Men/ "Reyes Magos" present, surely)
Could I work with a normal (not HC) 2gb microSD for the GW, and the standart 4gb SD the 3DS came with?