I swear, we used to have a topic with exactly this same title, but I just searched a 28 pages of this board with no luck, so maybe I've just lost it.
Anyway, the first step is to assume you can't write a character that's smarter than you. I know that sounds silly but I'm sure that's were 90 percent of the hang up is. Just imagine smart characters being portrayed in movies. What's his face definitely isn't as smart as Stephen Hawking, but he sure did play the role.
Next is to stop thinking that a smart person is going to have a smart person personality. That doesn't exist. People who are smart can have any kind of personality imaginable. Always start with a personality, then work your way backwards. If a smart character has a *censored* personality, then you might as well replace them with a search bar because all they're going to become is a mouth that spits out information.
Next, realize that a smart person doesn't have to act smart all the time. A lot of smart people act just like you and I during any given moment. The smart aspect only kicks in when something related to what they're smart in is happening.
Finally, stop using the word "smart" entirely. Erase it from your vocabulary. It's a worthless, reductive word that comes far short of actually describing what humans perceive as being intelligent.
Let me explain.
There's really no difference in intelligence between you, me, and the other 7.6 billion people on the planet (excepting obvious special cases). Some people know more about certain subjects than others, sure, but that is not a measurement of how "intelligent" someone is. That's just a tally of ignorance. There's a lot of stuff that I know that you don't know. There's a lot of stuff that you know that I don't know. It doesn't mean one of us is "smarter" than the other. It just means we have had different experiences than the other.
"But Coryn!" You may be thinking. "I always had a hard time learning in school, while the kids are next to me always got straight A's! Obviously they were smart right?" Well you would be wrong! Again, we all share the same capacity to learn (a good a definition for intelligence as I've ever seen). The difference between you and the honor student next to you isn't that your capacity to learn differed, but that they were suited to learning in the single environment public school sets up for us to learn in, and you weren't. That doesn't mean you're less intelligent. It just means that sitting in a desk and repeating whatever the teacher told you back to them is not the optimal way for you to learn.
The easiest way to understand this, is through what we've all heard before. "Some people learn by hearing, some people learn by doing." Personally, you can demonstrate something to me a 1000 times, but until I get in there and do it myself, it's never going to sink in.
So okay, I know none of that really answered the question you were asking directly, but it's all necessary to understand in order to write a 'human', which is ultimately what we're all going for here.