Interacting with people who think differently is a good way to start. Using the word 'moron' is a bad place to start.
I have very strong spatial thinking skills - I can imagine things in 3D and can intuitively understand how mechanical things work, but I'm very bad with numbers and math. My wife is nearly the opposite. She thinks in words and logic, but has problems understanding things spatially. She struggles to understand things that are obvious to me and I struggle to understand things that are obvious to her.
In other words, while we both are very intelligent, we're also different.
You might try understanding this by imagining thinking about something that's difficult for you to understand - take for instance the relationship between gravity and time (look it up... it'll blow your mind.) Then imagine what it would be like if listening to someone give directions to the nearest bathroom were just as difficult to understand. Then imagine if everything were just as difficult to understand.
Imagine the emotions you'd feel if everyone else could understand things easily while you can't.
And then you can probably start writing your character.