You can use the same computer; while the computer is still connected to the internet and you're logged into your normal OS, use Electrum to create a watch only wallet by importing your public key or address.  Once Electrum has synchronized you'll see all your coins there, and you'll be able to make a partially signed bitcoin transaction (.psbt) file.  Save the .psbt file to a USB drive other than the one you'll be using for tails.  Then shutdown the computer, insert your Tails USB and use that to boot the computer.  Import your private key into Electrum, then plug in the USB drive with the .psbt file on it.  Use Electrum to sign the transaction, then save the signed transaction file (.txn) to the USB drive with the .psbt file on it.   Once you've created the .txn file shutdown your computer again, remove the USB with Tails on it, and boot to your normal OS.  Then you can use Electrum to open and broadcast the .txn file.

You don't want to connect your computer to the internet while it's running Tails.  That could potentially expose your private key to the internet, which would defeat the purpose of having off-line storage in the first place.  Regardless, Tails' default settings won't let the computer connect to the internet anyway.  Tails is designed to be used off-line.  It does include connectivity features, but again, they are turned off by default.

Another thing about Tails is that it's not persistent, meaning it won't save any information such as the Electrum wallet file with your private key in it.  Nevertheless, I recommend using the terminal command shread to wipe the USB once your done.  If you're using Window the powershell erase command will write zeros to the USB stick, which is better than just merely deleting or formatting it, but it's not as good as the Linux shred command.