Requirements
Unmodified USA version z64 ROM of Mario 64: Other versions won't work, and the installation program will automatically reject them.
Installing Parallel Launcher
Switch to Desktop Mode: Switch to the desktop on your Steam Deck.
Open Chrome: Open a Chrome browser and search for "Parallel Launcher" on Google.
Download the Flatpak Package:
Access the site and click on the “Flatpak (SteamOS/Other)” link to download the package.
If you're following this guide directly on your Steam Deck, you can click https://flathub.org/apps/details/ca.parallel_launcher.ParallelLauncher to download the package.
Install Parallel Launcher:
After downloading the package, navigate to the download folder and double-click on the package.
Follow the instructions from the package manager to complete the installation.
Registering on Romhacking website
Create an Account:
Go to romhacking.com and create an account if you don't already have one.
Remember your credentials because you'll need them for Parallel Launcher.
Configuring Parallel Launcher
Select the ROM:
After completing the installation of Parallel Launcher, open it and select the folder where you saved the ROM.
Make sure the ROM is in z64 format and not compressed.
Enable Romhacking.com Integration:
In the Parallel Launcher settings, enable the integration with romhacking.com and enter your credentials.
Launching Super Mario 64 Maker
Access the Project Page:
Go to the Romhacking page for the mod project: Mario Builder 64.
Start the Game:
Click on “Play Now.” After a brief download, the game will start automatically.
SteamGridDB - Parallel Launcher