Journalling is a great tool to help with discernment. It allows you to keep a record of your thoughts and feelings throughout the process.
All you need to do is find a notebook or buy a fancy journal and regularly jot down or draw some thoughts or feelings that relate to your discernment. This could be quotes from talks you've heard, bible verses, song words or things you've thought of whilst reflecting or praying. There are no rules as only you and God will see what you've written (unless you want to share them with your spiritual director). The aim of journalling is to allow you to retain your thoughts and track your progress and development.
Give it a go and see how you get on!