You don't have to stick to doodling just flowers, puppies, or your own last name. You can start by doodling a garden of flowers, then think of your best friend Mary Flowers and start drawing her pet poodle, Pork Chop, who makes you think of the delicious pork chop dinner you had last night...just start with one image and keep drawing whatever pops into your head.
You don't have to stick to one theme or concept. No one is judging you -- and it's likely that no one will even see your doodles, so feel free to draw whatever you want.
Try to think about what it is that you want to convey with your doodles. Then, think of different materials and techniques that can help you with that idea.
Experiment a bit and always remember: there's no right or wrong when it comes to art. 

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