on that link i put, use the top right pic as your linear drawing, either print off and make bigger on a photocopier til the same size as your face tin,or, if you can't(like me) get a piece of baking parchment paper, draw round your face tin and freehand it on.
cut out the v head shape bit from your drawing, and lay it on the face bit.
have some royal icing coloured black in a piping bag with a small round nozzle. pipe a line on the cake following the v shape you've laid on it. I found this was good to be able to pipe the rest on in the right place.
draw on the rest of the features either winging it by eye, or cutting them out, laying on, and icing round them. remember if you bodge a bit you can wipe off and redo.
I then used the same icing to fill the black bits of the eyes, nose and mouth in.
I let it go hard overnight, but 5-6 hours might do.
I made up a batch of white fondant icing(it dries a bit harder than glace, though this would be ok too) coloured bits as i needed and filled in using a kebab stick to drip it in as it spreads.
recipe for royal icing here, though you only need about a quartef of that amount and don't need the glycerine.
fondant icing- buy in same kind of box, on shelf near normal icing sugar, mix like you would for glace, but with warm water in a bowl over a gently heated saucepan of water, not too hot or it will lose it's shine.
hope that helps, i can send a bigger pic if you like.