Like others said, follow the guide linked above to install Arm9LoaderHax. It's a long process (around 4~5 hours for first timers) so if time is the bottleneck, don't worry, the guide was made in a way that you can pause what you're doing and restart it whenever you want. Try doing half of it on one day and another half on another.
The hack basically is a payload that is in the FIRM partition on your 3DS, which loads miliseconds after the console boots. No glitched screens, no nothing, just a black screen into your SysNAND if you choose to go with Updated SysNAND mode or EmuNAND/RedNAND if you still want it. I personally prefer updated sysNAND mode since you can't patch this exploit with the current CFWs, you save 1Gb or 2Gb from the SD card due to the emuNAND and you only need to install GBA/DSi titles once which saves even more space.
Just an advice, read the guide carefully and don't skip steps. If you have any doubts, come here and people will gladly help you.