The passphrase is just a 25th word added to your seed, as the BIP-39 standard dictates. So, the idea is that even if your seed gets compromised, the wallet generated by only the 24 words is a different one than the wallet made by 24 words + passphrase.

To setup a decoy wallet, simply import just the 24 words and deposit a relatively low amount of BTC in there. Whoever stole your seed will think "I got it" and will stop looking, leaving the majority of your funds safe in the wallet unlocked by the passphrase. If something like that happens, it is still good practice to wipe the "real" wallet and move your funds to a new one.

Hope this helps!