I read that after you sketch the subject for a portrait you should transfer it on canvas by using transfer paper. What is transfer paper and is this the only way to transfer a sketch?" -- Soapid
Transfer paper is special type of paper coated on one side with either a colored wax or graphite. It's sold in art-supply stores by the sheet or roll. You can make your own by rubbing graphite (a soft pencil), charcoal, or pastel onto one side of a sheet of paper.
To use it, you place the transfer sheet between your sketch and canvas, go over your sketch with a pencil or blending stump (anything that applies pressure), and the transfer paper will mark the canvas accordingly.

