If you don't already keep a journal, it's a good idea to start. Here are some tips if you find yourself staring at a blank page.
Treat yourself to a beautiful journal and a pen. Choose a pen that feels good in your hand and writes comfortably. If you don't love your tools, you're less likely to use them.
Write anything. It could be a letter you'll never send, a description of what's tickling your five senses right now, a list of what you're grateful for and what those blessings have taught you, or a personal story involving an emotional experience and what you learned from it.
Don't edit as you write. Let the words flow. Ignore spelling and grammar mistakes. Don't worry about writing in complete sentences and don't judge your journal writing.
Make daily writing a ritual. If writing becomes a habit, then it's more likely to take priority over less-important activities. Dedicate 20 or 30 minutes every day to this practice.