Very sandy soils are often water repellent. Dry hydrophobic soils can be improved by initially applying a wetting agent. You can use a commercial product, or just soapy water, grey water irrigation also makes soils less water repellent. Another recipe is to dissolve 2 tablespoons of powdered agar agar in 2 cups of hot water to make a runny paste. Stir 250 ml of this paste into 5 l of water and water on the garden.
All these measures are only short term solutions, but necessary to get you started, once your soil is water repellent.
To improve your sandy soil add plenty of organic matter as listed above. On extremely sandy soil, layer up to 20 cm of this, mulch it, keep it moist for 2 or 3 weeks and then start planting.
Since sandy soil is poor in nutrients add ground rock dust. This contains trace elements which plants need only in really small amounts. Without it, however, plants may be stunted, less productive and disease prone. Microbes love it and they convert it into a form plants can absorb.
Mulch your beds to keep them cool and reduce evaporation. Keep the beds reasonably moist but don’t over irrigate. You need to find the sweet spot between not leaching out the nutrients and not getting back to dry, water repellent soil.
You will get an instant effect and can start planting. Sandy soil needs an ongoing effort to build structure and maintain fertility, it should always be kept planted. Don’t dig or til your garden deep, rather build a layer of humus over the years. Water retention and nutrient density will improve with every year that you keep on adding organic matter.
A mix of seaweed solution and fish emulsion will keep your plants thriving.