Letter series work like number series once you turn each letter into its place in the alphabet: A is 1, E is 5, J is 10, O is 15, T is 20, Z is 26. Those five landmarks make counting fast.
Then use the same routine as for numbers. Find the step between letters. If the steps change, look at how they change: they may alternate (+1, +3, +1, +3) or grow (+1, +2, +3).
Three twists come up often:
- Wrap-around: after Z the alphabet starts again at A, so X + 4 is B.
- Pairs: AZ, BY, CX pairs each letter with its mirror from the other end.
- Codes: B2, D4, F6 pair a letter with its position, so the next is H8.
Count on your fingers if you need to. Speed comes once the landmarks are automatic.