The last 7 bit of a 12 word seed are the checksum.

A checksum is to prevent typos. It is a way to easily know whether your seed is a valid one or not.

The probably fastest way would be to choose 11 words from the list, and then try out the 12th word. You need to try out 16 different words for the last spot ON AVERAGE to receive a valid checksum.


But.. please note that a human brain choosing those words from the list can NEVER be as random as a random number generator. You are giving away quite some bits of strength.