In Windows, right-click "This PC" > Manage > Disk Management. Shrink your existing volume to create unallocated space (20GB is ideal). Do not format it.
Restart your PC and boot from the USB. Choose "Install Prime OS." prime os 2.1.3
Looking for the download? Due to the project's discontinuation, official links are down. However, the Internet Archive (Wayback Machine) hosts verified checksums for the Prime OS 2.1.3 ISO. Always scan downloaded files for viruses. In Windows, right-click "This PC" > Manage >