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You'll get better at visualizing with practice. Making sketches lets you nail down the big picture without the tool and all of its detailed needs getting in the way.

Not the OP, but I swear by cardboard, hot glue, and play doh.

I make lots of rough scale "prototypes" with the above tools to get started, and spot lots of problems this way.

Everyone has different ways of thinking - I imagine the models before CADding them but some people might like to make cardboard, playdoh etc models. If you like going directly to CAD, I would recommend starting with coarse CAD models.