This is an important skill to have as a Product Manager. No, I am not saying Product Design = Product Management. This skill is about people in product management being able to communicate visually their ideas with their stakeholders to get rapid feedback. This makes what you are thinking real and tangible. Else you can risk conversations going in circles or different people interpreting the same thing that is said differently.
Ever heard of Show > Tell? So how might you start doodling?
TIP#1: Start with stick figures - its a start from writing paragraphs :)
TIP#2: Give the crazy 8 exercise try. Your doodles do not need to be pixel perfect like your designers
TIP#3: Start visualizing your user flows and customer journeys (there are ready to use templates in tools from UXPressia or Extensia etc)
TIP#4: Lo-fidelity designs are like a white-board doodle. Set up a free account in Balsamiq (there are others) and drag and drop to create low fidelity wireframe of what you might be thinking.
TIP#5: Heck, you can even put all of your low fidelity wireframes onto Invision to create a clickable prototype to do some early validation
Product Managers need to get the design they need in a product management way
A product’s intention is really a PM’s vision
Intentions are extremely hard to communicate
Product Managers, really help visualize a Product’s intention
Think of yourself as an enabler of design - your visualizations do not need to compete with a designer’s quality