(If you have not used a theme yet, go to the theme menu on sysNand and let it create the theme-management data, I'm also assuming rxTools in already in the /3ds directory of your SD card, also assuming you have an emuNAND set up)
1.) Download Homebrew starter kit from
http://smealum.github.io/3ds/ and unzip it to your sd card
2.) Scan the QR code at
http://yls8.mtheall.com/3dsbrowserhax.php with your 3ds (L+R to open camera, then click on the QR code symbol in the bottom left)
3.) From HBL, launch menuhax-manager and hit install.
4.) Turn off 3ds.
5.) Open SD card on PC and rename boot.3dsx to bootHBL.3dsx
6.) Download ctrBootManager from
https://gbatemp.net/threads/ctrbootmanager-3ds-boot-manager-loader-homemenuhax.398383/ and unzip it to your sd card
7.) Boot up 3ds with L button held.
8.) When ctrBootManager shows up select rxTools and start holding L again until rxTools loads.
NOTES:
If browserhax does not work, try clearing your cookies.
If you're on n3ds, rxTool's inject FBI does not work use this tutorial instead to inject FBI into H&S :
https://gbatemp.net/threads/release-inject-any-app-into-health-safety-o3ds-n3ds-cfw-only.402236/
I'm bad at writing tutorials