Light your candle, turn any other lights off and close the curtains. Once, long ago, this was how people used to write. In your head, go back in time, back to a time when there was only candlelight, it could be 100 years ago, it could be 1000 years ago, just go back into the past.
Still in your head, with the candlelight flickering, imagine the place you are in. What is it like? What are the walls made of? Wood, stone or brick? What textures are there - rugs, carpets, curtains, tapestries, furs...? Is the light from the candle reflecting off anything? Take a few minutes to really see the place you are in.
Now imagine what you are wearing. How do the fabrics feel on your skin? Are they heavy or light, rough or smooth? Are they cumbersome or streamlined? Restrictive or loose? How does it feel to wear your clothes? And now, move on from your clothes to your background - are you rich or poor? Do you have a job? How do you feel about what you do?
Try and fix this past version of yourself in your head then, when you feel confident, begin to write a letter to someone in the past.