Consider trees. Their biology and mythology. Their leaves and needles, fruits and shade.
Think about all the different things that can be made from trees.
Begin to write with a tree in mind and see where it goes.
You could make a list and comment on it, tell one or more stories from your life that include a tree, or record your ideas alphabetically.
Read over your work when you have filled a page. Illustrate if you like.
Think up a title and write it at the top of the page. Put your initials or signature somewhere on the page, along with the date.