Thank you so much for the suggestions. But I think it has been my mistake: I know more or less the process, first big details, making loops, cutting, separating to objects and so on. The problem is that if I start from an incorrect form of the body, I feel it will be more difficult to correct in the future.
And the most striking part of the bus and the one I cant figure out how to model accurately is this:
So Im stuck here. Yes I have tried to continue obviating that part, but of course I dont like the result and I think it will be very dificult to correct it in the future:
Starting with the correct body I think should be mandatory.
About the bus model I cant provide helpful information. The first pic is the correct one (that is the bus I want to model). The autor of the pic says it is a Pegaso 5520. Searching for that model none is concordant. Instead of that, the Pegaso 6035 seems to be more acurately.
But here comes the problem: the manufacturer (Pegaso) sold the bus without the bodywork. So it differs a lot from states here in Spain. My state bougth the bus and gave it to a factory to build the bodywork according with some determinate needs. A lot of diferent models with the same base.