a simpal way is to devide the pictur up into whats known as primatives. thees are the simpal shaips inwich every structur is comprised of.
circlas squairs etc etc
Then after you astablish the basic shape with the primatives build up detail. start with basik line drawings. then observ howe light reacts with its suroundings before replicating it on paper. dooing sharows well is dificald
sorry the pic is so big this pc dusant have what i need to shrink it