What are you trying to do?
If you are thinking you can just make your own seed by thinking of 12 "random" words, it won't work. BIP 39 is much more than just a bunch of random words, it is an encoding for a random number. It includes a checksum, so you can't just think if 12 random words and call that a valid BIP 39 mnemonic. It is far better to use a wallet that supports BIP 39 to generate the mnemonic for you since they actually generate the random number first and then encode it as a mnemonic.
stefek99, perhaps this might accomplish what you seem to be trying to accomplish:
Generate a BIP 39 mnemonic for, say, bitcoin and then use that same seed phrase on other wallets for alt-coins. So you could use one BIP 39 wallet to generate the first seed phrase and then restore that same seed phrase in your wallets for other coins?
As far as whether that would work, I haven't tried it myself, but if the alt wallet and alt-coin haven't altered the spec, it could work. [Just be sure to use a non-backdoored wallet on each alt, otherwise you could lose your coins everywhere.]