I am sorry to say Wolfy, that this is not an easy question to answer:-(
"How to Sculpt a helmet?"
1: Is this for a one off- or for casting?
This answer will help massively, as if it is a one off, then you can base the 'structure' of an existing model/ object. In this instance, I would be looking for an appropriately sized head or alternative, with which to adapt/ cut down/ resculpt over.
If it is for casting, then depending on the quantity you want you can go with the above approach, but I believe our 'line' on this is that if adapted from an existing model, then no more than 10 cast for private use will keep you away from legal snipers. If it is more that is needed, then scratch built is the only way.
2: Next, I would source inspiration images (always with the research!) with which to base an idea on- or sketch one out. Your images will suffice for this purpose as they give a good idea of the 360 view.
3: Then- is there any details that I can take from another model and incorporate here- for example, the main eye piece, the 'ear' baubles. In your shoes, I would be thinking that I can take the lense from almost any kit such as the marine devastator sergeant back pack, slice off and stick on the front of an empire halberdsman helmet. The baubles could be taken of a templar censer hanger...? Always be thinking of parts that can be cannibalised.. GW actually make this process very easy as much of their design is of an archtype- ie 'big statements' that reverberate around their races. Hence Jes's work (marines and tau) have strong visual keys, eldar, elves and necrons have similar line etc.
4: Sorry, but, you have procrastinated enough now... get the putty out!!
Regardless of whether you use S, SS, GS or MP (see the sculpting lexicon), get your dentist tools and clay shapers out and start pushing it about. Without going into specific sculpting tips here, it is just a case of play and practise. You will learn resistances, tackyness and how well the putty carves, sands and cuts (OR NOT!) when cured. Only when we see how you have got on with your first attempt can we assess your individual needs with this very deep- topic!
I really like the design of this, and so very much look forward to seeing how you get on!
I hope that this helps?
cheers
W