The first polyalphabetic cipher — 26 stacked Caesar shifts in one table. Trithemius's own method used a progressive key: the shift simply increases by one for every letter, unlike the later Vigenère cipher, which repeats a chosen keyword instead.
How the shift progresses — the 1st letter shifts by 0, the 2nd by 1, the 3rd by 2, and so on, wrapping back to 0 after 26 letters — reading straight down the diagonal of the tabula recta below. Non-letter characters are skipped and don't advance the count.
The Tabula Recta — 26 Shifted Alphabets