Start with comparing the codebase of Litecoin with existing Neoscrypt-based coins, like Feathercoin or Phoenixcoin. I don't know if there is an existing detailed step-by-step how-to guide about it, but seems you might have to do it on your own.
Phoenixcoin codebase looks like is the closer that you could get since it is also forked from Litecoin.
https://github.com/ghostlander/Phoenixcoin, try to look at which part of the codebase they do change and better if you comprehend what the code actually does, so you just randomly change the line.
