How to do this?
There are a couple of ways. One, for characters who play a significant role in my story I go online and find pictures of what I think that person looks like. Now I have a concrete visual that I can use in description.
Another way is to find out things about the character, even if it does not seem relevant to the story line. What job does the person have? What sports do they like? What things do they carry around in their pockets? What music do they listen to? The more I know about my characters the more real they become and the easier it is to make them come alive on page.
As a very minor example. In my newest book, Honestly, Ali, leaves the headmaster's office and runs past the secretary at her desk. I could have just left it at that. It's a minor point, but I describe it this way.
I hurried out, through the outer office, past Mrs. King who always had a smile for everyone and a big bowl of candy on her desk.
This tells us something, however small about Mrs. King and makes her more real.
A tactic I use with major characters is to have them tell me who they are. I assign them an essay entitled "Who I Am". What the character tells me in that essay doesn't necessarily have anything to do with the story at hand, but it gives me a better feel for who the character is over all.
For example, the fact that although Ali's mother is a movie-star, Ali is fashion-challenged, is a running joke throughout all the Ali books. In fact the first book starts with Ali realizing that her Mickey Mouse t-shirt may not have been the best choice to wear on that particular day. It's a minor point, but it helps to keep Ali real throughout the books.
What tactics do you use to make your characters come alive?