How do あなた base a character around a real person?

This is done in many ways. Sometimes あなた might name a character after a real person または just give him/her that person's look. But I guess あなた mean giving a character a personality of a real person. :)
First of all, あなた have to know the person really well. Decide what exactly あなた find interesting in him (or her), what trait is the one あなた want to replicate and why.
Remember, basing a characters on real people doesn't mean that they don't need background. Also, don't be afraid to add your own ideas.

Characters, loosly based on people are quite common to the world of fiction and if done properly, a charater can seem very realistic.

Good luck with your writing. ;)

This is a great idea!
First, if the person is a celebrity または something, do research. Make sure あなた know every little thing about the person you're basing it off of. But make sure not the EXACT same thing happens in the life. Anyone can write a biography and switch names around, but not everyone can add their own twists.
I'm going to be really honest in this paragraph...sorry if it sounds harsh または something, I'm just trying to be helpful! Anyways, it really depends on what あなた want the person in your story to do. The only thing I know about Jensen Ackles is that he's an actor. Not sure if that is a great idea because if あなた just go around 芝居 and signing autographs (not that that's a bad thing, it's just not very interesting in my point of view), it's not a real interesting book. However, maybe there's something I don't know about him...he went through a hard time in is life または something. Then that could be interesting.

Usually if I do this, I pick an older person who already died to base a character off of...an old クイーン または something, for example. Someone people don't really know about is cool! If people know a ton about them, they can just pick up a Teen Beat または something to read.
But really, あなた can do whatever あなた want!