Answer:
The best way is to contact the mission headquarters for the mission where they are serving or did serve. If they are currently serving, they can give you an address to send a letter. If they have already gone home, they may be able to provide a home address. If mission headquarters doesn't have any information, many missions have alumni websites. You could search for your missionary there. You could also try checking out mission reunions, which are usually held in Utah in April and October. A final way to search for a missionary who has gone home would be to look for them in local phone directories or on social networking sites such as Facebook. Don't forget to try to contact other people who might know the missionary, often they will have information as to how to find your missionary. There are some "Related Links" below that might help you out.
Or, if you meant the other definition of trace, have the missionary lay down on a very large piece of paper, and carefully follow the outline of their body with a pen, pencil, marker, or other drawing material.