Using An Offline Computer
When generating a wallet on Tails Offline with a tool like Ian Coleman's Bip39, remember to use a room with little to no signal or Internet connect (basement is best).
Remove all electronics and phones from that room. Only the laptop or desktop should be present.
Use a computer that is brand new. Though you are most likely booting Tails from a USB Offline, a clean or blank/formatted computer can go a long way.
Do not use a wireless mouse or keyboard (those signals are not encrypted).
Use fresh SD cards (not USBs or MicroSDs) for data storage and transfer into and out of your safe offline environment. They are usually simpler than USBs and you have less of a chance of being infected by malware on these chips (however, it could/and still is possible, so be careful).
If you are storing or backing up seeds, keys, passphrases or unencrypted KeePassX databases, never plug those into a live online computer (ever). If you have accidentally done so, assume those keys are compromised and remove all funds from those accounts immediately.
Keep your Offline Computer offline. Use another computer (preferably one with Tails OS installed) for browsing, unsafe surfing and file downloading. Remember not to have your secure or sensitive SD card chips inserted while doing this.
If you want to transfer data or software to one of your secure SD cards, please follow the instructions in "Safe Data Transfer Between Secure and Insecure SD Cards Offline".