This may seem strange but i use a medium size sheet of tracing paper. Mix the same color which was used for the imprimatura or first layer. Spread a very thin layer onto the tracing paper. be sure there is not too much paint or it will cause messy dots all over the canvas. place this paint side down and put your image on top. Use whatever works to go over the drawing, being sure not to miss anything. Always be sure to mark your image and canvas in order to realign it if necessary. i will try to take another photo for this demo.