How do you get 24 volt out of a 12 volt transformer with 2 12 volt terminals?

If there are just two 12 v terminals you can only get 12 v.

If there are two equal 12 v windings (two pairs of terminals each providing 12 v) you can connect then in series to get 24 v. It is important which way round the two windings are connected because one way you get 24 v, the other way you get nothing.