My advicewould be to cheat! I started out tracing things from magazines, portraits etc before doing it for real. So use tracing paper to outline the image, then draw on the reverse side to match these lines and then use a pencil to transfer to your sketchbook or paper - then you can concentrate on tone and lines and not worry about scale! I know its cheating but its good practice!
Some examples here
mawganart.blogspot.com/
(Some examples here
mawganart.blogspot.com/
Mster Johnny Ekstrom, now that's a name,
Wed 2 Dec 2009, 17:18,
archived)
Orrrrr....Use the gridding method.
(
Jamie_W If your life had a face I would punch it.,
Wed 2 Dec 2009, 17:49,
archived)
like in The Draughtsman's Contractyep
( Mster Johnny Ekstrom, now that's a name,
Wed 2 Dec 2009, 17:50,
archived)