Step 1: Improve water/nutrient retention
The hardest part about farming a desert is not the heat or even the salt. It’s the inability for the sand to retain water and nutrients. When you place fertilizer in pure sand, and then add water, the water causes the fertilizer to wash away leaving you with inert sand which most plants can not get enough micronutrients out of to survive.
The best way to solve this is to import clay which you can mix into the sand to create lome. When water mixes with clay it makes the clay sticky and bind to the sand allowing water and nutrients to be retained better.
Since this is a desert, you will probably also want to add charcoal to the soil. It will further increase the soil’s water retention and add small amounts of certain biologically available nutrients to the soil to help get things kickstarted.
Step 2: Fix soil acidity
Deserts tend to be far too alkaline for most crops, and most agricultural plants make the soil even more alkaline. Many deserts today exist because of bad farming practices depleting soil acidity; so, you need to plant starter crops that will help with this. If you plant olive trees, you will produce an agriculturally valuable crop that will naturally fertilize the soil with biologically available Nitrogen and its wide, deep roots will further help with soil retention. Olive trees are very long lived, and thier oil is a major cash crop; so, if it were me, I’d stop here and start raking in the profits. But if you are are hard set on cereal crops, you could chop and drop them, and after a few more years of decomposition and fallow, you will have great soil for growing your cereal crops.
But since you want cereal crops, there is a much faster solution. Instead of the Olive trees, you could add sulfur to soil. In the real world sulfur is only a so-so solution because it still does not address the extra salt or other scarce micro neutrants, but it does get the job done, and you can solve the extra salt with magic; so, who cares. It will acidify the soil making it good enough to start growing stuff, though your soil will be far less stable than if you have those wide deep olive tree roots left in the ground holding everything together
Step 3: Keeping the soil productive
One of the big problems with agricultural desertification is the lack of good cover crops. We all know that wheat takes up the nitrogen and legumes put it back, and some other thing can be planted in winter to prevent erosion, but soil is more complex than this. You need a mix of cover plants that can scavenge micronutrients from that sand, build biomass, aerate the soil, encourage microbial growth, and attract pollinators, etc. One mix that’s been gaining popularity lately is the 5-Cousins method: Amaranth, Cowpeas, Buckwheat, Daikon Radish, and Sorghum. Sown together, these guys will build up your desert soil year after year into progressively better soil needing little to no outside fertilizers and a minimum of water.