As long as you have rocket Launcher set up correctly and can launch your games directly in it then the only thing in Launchbox you need to do, if you are planning on just using Rocket Launcher, is to make sure the games in the platform in Launchbox are set up with Rocket Launcher as the emulator. Of course, you need to also make sure you have Rocket Launcher as an emulator in Launch Box.
Right click a game and check the emulation tab. If it is not set to Rocket Launcher you need to change it to Rocket Launcher. You can edit all games for the platform by clicking on one and hitting CTRL+A. Then choose edit and in the field drop down box choose Emulator. Select Rocket Launcher.