Hello @queenofdisasters 
Francisco shared some great advice above- I agree, it’s best to start small and practice, practice, practice! 
Adding to that, you can also help improve visualization by using art 
Painters, drawers, sculptures, writers, and artists of all kinds use visualization to make masterpieces- their tips and tricks and really help you to paint your own (mental) pictures! 
Have a physical object in front of you. Studying closely, and try to memorize the details. Then take a blank piece of paper and try to recreate that object- either drawing it, painting it, or writing out a description with words. Look back at the object anytime you forget. Doing this can help your mind learn how to visualize things without needing to rely on your eyes all the time! 
To help get you motivated, I recommend checking out the discussion about Art Witches 
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Good luck and blessed be!