This guide assumes you have access to RCM, if you do not, use a compatible method. Some of those compatible methods can be found here.
This guide will transform your stock switch to play "backups" of switch games
1.) Installing ReiNX

  • Download the latest stable ReiNX Release (Found here) (Extract this on to the root of your SD)
  • Make a new folder in the /ReiNX/ directory on your SD named 'titles'
  • Download hbmenu.nro (place this on the root of your SD)
  • Download this .zip file and extract it to /ReiNX/titles/
  • Download the latest stable release of Tinfoil(Found here)
  • Create a new folder on the SD's root named 'switch'
  • Place tinfoil.nro (found in the tinfoil zip) in the /switch/ directory
  • Make a new folder on the root of the SD named 'tinfoil' and inside this folder create another folder named 'nsp'
*From now on any NSP warez or homebrew you download should be placed in /tinfoil/nsp on the SD card.

2.) Booting into ReiNX
*Skip this step if you are familiar with booting into CFW
  • Use your method of choice to enter RCM
  • Push ReiNX.bin from the ReiNX.zip we downloaded earlier to your switch. (If you do not know what app to use to do this i reccomend TegraRCMGUI for Windows users.)
3.) Installing Games
  • Open the homebrew launcher by opening the Nintendo Switch Album
  • Open tinfoil
  • Choose Title Management>Install NSP>(Choose the NSP you would like to install)
Profit! Enjoy your "legally obtained backups" ;)

FAQ:

- Can I delete .NSP files from my SD after they have been installed via tinfoil?
Yes you can.

- Will I get banned for going online?
Yes, you will most likely be banned from playing online with your switch. This in fact does not block updates though, and you can still access the eShop. (I strongly suggest you keep your switch in Airplane mode and de-link your NNID if you plan on using CFW to install a bunch of NSPs.)

- Can I install updates or DLC with this tool?
Not with tinfoil currently, you have to install devmenu to install DLCs and game updates.

- Can I convert .XCI to .NSP?
Yes. Use this.

- Do i have to use the album to launch into the homebrew launcher?
No. You can install this via tinfoil and launch the homebrew launcher as if it is an app if that pleases you.

- I wish i could backup my saves so that i don't lose them...
Download checkpoint.nro and use it to backup your saves/import saves from another switch.

- Can i install any homebrew to my main menu with this?
Pretty much. Check this out.

- How do i update ReiNX?
You can update it manually by downloading the latest release and extracting it to the root of your SD card, or use this updater homebrew application. (Recommended you use that homebrew to decrease hassle every time you want to update.)