Well, I have a confession. My upcoming book, How to Run: an Advanced Guide on Managing Role-playing Games, does not contain a section on 'how to role-play.'
I considered this for a long time. I realize that there are DM's who don't know how to role-play, who find it daunting, who even feel silly while doing it. They would probably like some sort of guide that told them how to adopt voices or speak with conviction, or how to act out the part of a noblewoman as opposed to a char lady. No doubt, they must believe there is a 'trick' that can be taught.
The reality is that any advice that I could offer along these lines would be trite and unsatisfactory. Advice that I've seen in the past tried to point to examples or situations: if you were a dragon and you had lost everything to a party of adventures, how would you feel? Or the advice is very general and therefore meaningless: have attitude; make it interesting. And so on.
I saw little value in pursuing this course of instruction. The opportunity for people to initiate the learning process on how to role-play came when they were children, when any L-shaped object served as a gun, when a set of playground bars served as a spaceship or a castle, when an empty field covered in snow was a distant planet and so on. I can only express my sympathy for readers who wish to play D&D but did not invest this very important effort at an early time in your lives. It must suck for you.
I recommend having children. This is not something I'm likely to put into a 'how-to' book so I'll say it freely here. Make yourself tolerable enough that a member of the opposite sex is willing to - at the very least - fraternize and give you occasional access to the offspring. Obviously, you might want more than that, but occasional access will at least teach you how to look at the world as a mystical, marvellous place. That is, if you're able to communicate with children unlike a parent with his or her head up your ass. You may think that's obvious, but I remember being a child and a parent that could breathe air that was not coated in brown seemed like a very rare thing. So in following this advice, I suggest not getting 'stuffy' with your head. This will enable you to observe, and participate, with a human being that will be able to both teach you how to 'make believe' and tell you honestly when you're really shit at it.
I do not recommend attempting to make believe with someone else's children. Just don't go there.
Very well, you've received basic instruction on how to role-play. Here's what you're going to have to do next. Enjoy what you've learned. Recognize the 99% of those around you that are your own age have already decided this is either impossible or emotionally retarded. Try to view these people as your child instructor would - or as you would, if you were a child. Perspective is very important. Try to have some. Try to grasp that stringing a bunch of words together that are creative is 'childish' and that 'childish' is a wonderful thing. To be 'like a child' is to be inventive, suspicious, immersive and free from 'head-up-the-butt' syndrome. Children demand proof of friendship every day and will make instantaneous enemies of people who are selfish, miserable, abusive or otherwise impossible to get along with. Children like to learn. Embrace these things.
Step three. Rid yourself of everyone around you that cannot embrace these things. Just get rid of them. If they cannot accept your being a free spirit an hour or two a day - if they don't know the meaning of the words 'free spirit' - then show them the door. These people have already adopted death of mind as a preferential condition to life. Cut them out of your life. Their influence on your thoughts will only go to bad places.
Very well. Your players should only consist now of people who can embrace role-playing. You should be able to participate in role-playing games knowing you are surrounded by other people who embrace them. Enjoy.