For each month create a page with just lines for each day. Your goal is to write one sentence every day based on something you witnessed in nature. This practice may sound simple but it is powerful for a few reasons.
It builds a habit and eventually an identity.
Opening up your nature journal daily is a powerful practice that gets you in the habit of routinely using your journal. The more you use it the more natural it will feel. By engaging with your journal daily it will become part of who you are not just something you do. "I am an avid nature journaler."
It challenges your mind and makes you more creative.
When you challenge your mind to make a new observation every day you'll have to begin to see past the obvious observations, you'll use your creativity. Of course you can always just write about the weather but it's more challenging to stay away from routine observations, think outside the box!
TIP: At the end of the day it's about going through the motions of writing in your journal not about being unique everyday. Write something each day even if it doesn't seem particularly interesting.
This activities power comes from the momentum you build. Instead of writing just one sentence about what you observed you're going to want to write more... and draw more... and eventually you'll have an insatiable curiosity for life and a practice that will serve you well.