Set the key length, then shift each column until the plain text reads.

The top line is a safe bet: the commonest letter in a message this long is E about nineteen times in twenty. Below that, the right-hand column is only the general shape of English — useful for narrowing a guess, not for settling one.

Guess a word. Positions whose letter pattern cannot match are ruled out.

Everything you have read on previous days. Known phrases can be dragged as cribs.