Collect all of your play doctor supplies. If you don't have any, improvise. Use headphones for a stethoscope, grab a real thermometer, and use a spoon for a mallet. You can also use a couch as your doctor's table.
Set up your pretend doctor's office. It's a fun idea to have a reception area. A little card table with chairs for a waiting room is fun. In fact, if you have more than one child, the others can work reception while the doctor sees patients.
Invite your child in as your first patient. Shake his hand and introduce yourself. Go through all of your pretend doctor instruments and tell him what they are used for and what you will do with them.
Instruct her to tell you where it hurts and have her lay down on the table. This step is a great way to encourage the use of imagination. The crazier the illness or injury, the better.
Switch roles. Let your child take the role as doctor after you have had your chance. It's good to start out as the doctor to show your child how to play and what a doctor does. Be a good patient and come up with multiple injuries for your child to fix.