You have no need at this stage to be neat, really whack it on and build up the important parts and always make sure its symmetrical. Work the clay smooth with wet fingers until its a rough shape of Ironmans helmet.
Once this is done draw on the edges that have to be sharpened up and add more clay to get the right angles your going to need in order to sand them to the right sharp egde shape after. Dont be too fussy as this clay sand really well.
Wait for the clay to dry completely then just go over the whole lot adding little bits of clay to fill in those little holes you think wont sand out and he should look something like the pictures below.