Planning & Design
While it can be fun to jump right in and wing it, marionettes often need a lot of thought, and planning about the character you are building.
It helps tremendously to know the gender, age, weight, and athleticism of the character you are creating because so much about what an audience understands about a character comes from their movements.
You can communicate a lot about your character through their movements and that all begins before you've ever made a single cut with the carving knife.
Thinking about character
I recommend beginning with a drawing, even if you aren't good at drawing. It helps to create a portrait of the character that you can look at and refine, so you will know as much about them as possible going in.
This might include writing a backstory and coming up with details about their attitudes and philosophy. After all, a shy character will walk and move very differently from a confident one.
Even if this puppet is a portrait of a real person, you will need to understand a lot about their personality in order to convey that through their movement. We'll talk a lot about this in later chapters when we talk about weight, stringing, and balance.
Suffice it to say, now is the time to get all of those character traits as solidified in your mind as you can because it will effect every stage of creation.