https://en.bitcoin.it/wiki/BIP_0039#Generating_the_mnemonic

12 word keys contain 128 bits of entropy and 4 bits of checksum

to do it all offline first you need a good entropy source, like 128 coin flips, and then you need to take that number and compute the checksum which is the first 4 bits (single hex character, in fact) of the SHA256 hash of the 128 bit value

making the raw key in hex is probably easier than computing it into a valid word mnemonic, since you just group your bits into 4s and change them into 0-9a-f to get the hex format

i believe that this is one of the things you can do with a seed signer - take that hex, or even binary string and change it into a word key

assuming you can find an old, airgapped computation device that has an implementation of SHA256 you could use that also, most scientific calculators understand hex, just not sure how many of them you can program to do a hash