You will find method and techniques for Letter Series questions on this site, just follow this link
http://www.elevenplusexams.co.uk/pdf/th ... hnique.pdf
Letter Series start on page 30, there is an explanation followed by examples then a link to free demo questions.
One tip we have used with students who struggle with this type of question is to get them to use the methods and techniques to create their own questions. Once they understand how the questions are created they can better understand how to work them out.