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.