Migrating instances from ProjT Launcher
In Prism Launcher
In order to transfer an instance, or multiple instances from Prism Launcher to ProjT Launcher, you must begin the process by opening Prism Launcher on your computer.
Now that Prism Launcher is open, navigate to the Folders tab located on the top menu bar, to the left of the Settings tab.
Left-click on the tab and select the View Instance Folder option. This should then open your system's file explorer application with the correct directory in view.
Now that the Prism Launcher instance directory is open, you may select, and then copy the instances that you wish to transfer.
In ProjT Launcher
To complete the transfer process, you must now open ProjT Launcher.
As you did in Prism Launcher previously, navigate to the Folders tab located on the top menu bar, to the left of the Settings tab.
Left-click on the tab and select the Instances option. This should once again open your system's file explorer application with the correct directory in view.
Now that the ProjT Launcher instance directory is open, you may paste the instance(s) that you previously copied from the Prism Launcher directory.
Optional: If you would like to transfer your pack icon selections, navigate up from the Prism Launcher instances folder to the main Prism Launcher folder. Do the same for Prism Launcher. Copy the icons folder from Prism Launcher to Prism Launcher.
Finishing up
Now, in ProjT Launcher, your instances should appear as they normally would in Prism Launcher.
If you have transferred instances across devices or packaging formats, you may need to change the version of Java that'll be used to launch Minecraft. You can learn how to do so on this page.