Just think about it in your head, maybe if you lie to someone and you’re not sure if they bought it or not, you replay it in your mind, like that. Or if you talk to a girl/boy you really like, afterwards you might wonder if you said the rights things and impressed them or not.
When you walked out of your front door this morning, imagine as clearly as you can walking towards the door. You reach for the door handle, what does it look like? How does it feel? How does it sound when you turn the handle? How does it feel when you open the door? What can you see when you walk outside? What can you hear? Can you feel the sun shining or the cold biting?
You should do both at the same time. Run through your last dream by experiencing it again; see, hear, smell, taste, feel what happened in the dream again.
Edit: Ah, I see I’ve just repeated what others have said