Ensure your console has the latest sigpatches installed. Without these, the Switch will fail to launch custom NSPs.
If the .rar contains multiple files, install the Base Game first, followed by any Updates and then DLC. FUSER-NSP-ROMSLAB.rar
The .rar file is a container. Use a tool like or WinRAR to extract the contents. Ensure your console has the latest sigpatches installed
You can install the extracted .nsp files using a homebrew installer such as , DBI , or Awoo Installer . install the Base Game first