When I think of the tree planning phase, I think of a large tree, surrounded by dwarf trees, then surrounded by other plants, etcetc. There's a good picture of this here.
It's super important though to think of how big the center trees will be at maturity though! It's better to have your footrest look a bit sparse in the beginning to enable it to be fully functioning at maturity.
You should also test your soil for pH and nutrients. that can help you decide what you should plant to start enriching the soil. With pastures, I think people do "chop and drop"- where you now everything down then lay a bunch of seed down on top of it and repeat as necessary. Another way is to put a liner on top of sections until the grass dies and then to seed\mulch tghis. In the states, we can easily look up plant growth zones like 7b or whatever to help you figure out what can grow in your area. Can you find something like that for Spain? Also, does the government have local agriculture offices that can provide information and support?
ETA- you should check out Sepp Holzer.