Building Your Offline Air-Gapped Computer
Many of us don't have the funds to go and purchase a new (straight from the factory) computer to use solely as an offline air-gapped computer for Ethereum...but you can refurbish an old or used computer as well.
If you can, remove the networking/wifi card from the old/new PC or laptop.
Try also completely formatting or removing all hard drives from the computer. You only need to boot Tails off a USB; the hard drive(s) are not necessary.
Use a room with no signal or connection always (to ensure to connection attempts cannot be made by things like malware).
Remove all wireless enabled cards like BlueTooth cards.
Only use wired keyboards and mice (if needed, no wireless).
Cover over the mic/camera of the laptop.
Ensure no electrical or recording devices are around or in the room.
Keep the computer away from others, so they do not attempt to use or boot it accidentally
Use the same Tails USB: Only use the offline computer to boot into the same Tails USB offline. Don't plug that Tails USB into any other computer or device.
The only data transfer device that should enter the computer are SD cards. While highly unlikely, the SD card readers can be infected with malware. So do be careful with these SD cards.
Make sure you use the Tails Installer on a Mac or Linux machine to write the Tails OS. Windows is generally a mess of potential malware vulnerabilities and can potentially write malware to your USB stick or reader firmware or SD card reader firmware.
Note that classic PC towers are preferable to laptops.
You may also choose to record the MD5 checksum before loading onto the SD card, and re-checking the checksum in Tails with GtkHash before using or loading the software or file.
Assume the SD card chip and reader itself is trying to trick you and is logging every piece of data transferred on and off the cards.