Not all indexes are created equally. This is especially true with those that were created by local records clerks. Don’t just turn to what you think is the correct section of that printed or handwritten index without first familiarizing yourself with the organization and layout.
Some indexes are by first letter of last name only. Some separate out different sections of last names by their first two letters. Some partially index names by the last name and the first name.
Practice with the index if it is one you have not used before. Find a record at random in the series and see if you can find an entry for it in the index. Ask someone who has more experience using it or has more research experience in general.
Practice makes perfect and not learning how something was created reduces the chance you are using it effectively.