Put pen to paper
Definition 1 of 2
Pro Tip 1/3Starting Act
The phrase often implies the beginning of writing something significant or a long-awaited start. 
After months of research, she put pen to paper on her thesis.
Pro Tip 2/3Figurative Use
It can be used metaphorically to mean the start of any creation process, not just writing. 
The artist put pen to paper to draft his next masterpiece.
Pro Tip 3/3Deliberate Action
Suggests a thoughtful or purposeful action, not a casual or unplanned writing. 
She put pen to paper after carefully outlining each chapter.