Tubehax is an exploit in the 3DS's YouTube application that allows people to load 3DS homebrews. This works on firmware versions 9.0.0-X up to 10.1.0-27. Note: This exploit no longer works due to a forced update to the YouTube app.
First, you should download the 3DS Homebrew Starter Kit. When you've downloaded it, extract it to the root of your 3DS's SD card (SD:\).
Then, follow these steps:
1. On your 3DS, open the System Settings app, then go to Internet Settings and Connection Settings. From there, select your favorite Connection, tap the Change Settings button, and on the second page head over to the DNS section. Once there, select "No" for "Auto-obtain DNS", and under Detailed Setup, enter the following address : 107.211.140.065. Save these new settings and close the System Settings app.
2. Open the YouTube app. It should redirect you to a firmware selection page. Under "Select your firmware version," choose your console's firmware version. Getting this right is very important! Once you're sure you did it right, hit the Submit button. In case you don't know what firmware your 3DS has, it's on the bottom right on the top screen when you first open the System Settings.
If you did these steps correctly, you will be at a screen where there will be a glitched square at the left and a "Change Firmware Version" button on the top right. When you're at this screen, just wait and the homebrew launcher should load. Sometimes it will say "an error has occurred, forcing the system to restart" but just ignore that, and keep trying to load the homebrew launcher. It should work after a few tries.