By FlameKat53 & Epicpkmn11

Basic Setup

Get some .nds homebrew. (You can get some here)
Name your .nds files boot#.nds with # being one of these buttons; A, B, X, Y, Start, Select, R, L Left, Right, Down, Up, or Touch.
Put all of your files into /_nds/Relaunch/extras/, if you set custom paths in the .ini file you don't need to do this.
Boot Relaunch and hold the button corresponding to the file you want to launch.

Advanced Usage

You can edit the Relaunch.ini file and set custom paths, names, and file extensions for your files.
If you read the line above, you know about custom paths, you can do this in the Relaunch Menu as well!
Hold A + B on boot in order to open the menu.
There are prebuilt nightly builds of Relaunch found on TWLBot

Credits

Flame: The main developer of Relaunch
nocash: Creating Unlaunch, which this app is inspired by.
RocketRobz: Creating GodMode9i, the base of the Relaunch Menu.