A USB A to USB C cable, these usually come with switch pro controllers.
An RCM Jig or a paperclip.
Booting into Custom Firmware
This guide only covers one method of launching CFW, a tutorial that covers other methods can be found here.
Download SDFiles then extract and place it's contents in the root of you MicroSD card. Place "hekate_ctcaer_x.x.bin" on your desktop or somewhere that you can easily find it.
Make sure your switch is turned off. Insert the MicroSD card. Remove the Right joycon. Either insert your RCMJig or use the paperclip to bridge pins 1 and 10 on the inside of the joycon rail. If need be pins 9 and 10 or 10 and any piece of metal on the rail, such as a screw, can be bridged.
With the jig or paperclip inserted hold the volume up button the switch itself then press the power button. If all goes well this will boot the system into RCM. If the system is in RCM it will not display anything on the screen, if it starts displaying then you need to turn the console off, make adjustments, and try again. If need be you can restart the system by holding the power button for about 15 seconds.
On your PC open TegraRCMGUI. Navigate to the "Settings" tab then click "Install Driver." This allows the Switch and PC to communicate.
Once the system is in RCM plug the USB C cable into your switch and the other end into your PC. Switch back to the "Payload" tab in TegraRCMGUI. under "Select payload:" navigate to your "hekate_ctcaer_x.x.bin." Then click inject payload.
Once the payload has been injected select "Launch" then "CFW" using the "power" and "volume up/down" buttons to navigate. This launches the system into Custom Firmware which will allow you to play games with mods or load custom programs.
Download your desired mod.
Open the MicroSD card. In the root should be a folder named "atmosphere" or "sxos"
In "atmosphere" create this path: "/titles/(your game's titleID)/romfs/" For BotW the Title ID is "01007EF00011E000"
Open the mod you downloaded, it will either have the above path prebuilt or it won't. If the path is built copy it to the root of your MicroSD card. If the path is not present copy any folder present to the "romfs" folder.
Once all the mods have been copied over put your MicroSD card back into your switch, launch CFW, then boot up the game and your mods should load.