Check that the old computer powers on and has a working motherboard, CPU, RAM, and storage drives
Back up any important data from the old computer
Choose a NAS operating system such as TrueNAS, OpenMediaVault, or Unraid
Download the NAS operating system ISO or installer
Create a bootable USB drive with the installer
Connect the old computer to a monitor, keyboard, and network cable
Enter BIOS or UEFI and set the USB drive as the first boot device
Boot from the installer USB
Install the NAS operating system onto a separate drive if possible
Reboot into the NAS operating system
Log in to the web interface from another computer on the same network
Set a static IP address for the NAS
Add and format the storage drives you want to use
Create a storage pool or array
Create shared folders or datasets
Enable file-sharing services such as SMB for Windows, NFS for Linux, or AFP if needed
Create user accounts and set permissions
Enable remote access only if needed and secure it properly
Turn on SMART monitoring and disk health alerts
Configure automatic snapshots or backups if supported
Test file access from another device on the network
Connect the NAS to a UPS if available
Keep the system updated regularly
Replace any failing drives as soon as possible