You probably heard before that large forests alter the weather around them. This is no exaggeration. Deserts are locked in a state of infertility because ay rain that falls quickly washes away, taking with it any top soil. So first you big holes to trap water underground, then you let the biomass build up some top soil, then you plant trees. When you have enough trees, they start pulling water from underground into the air, increasing moisture and starting a positive feedback loop. Then you start expanding into the surrounding desert areas. It's a lot of work, but possible, and being done.