Preparing the soil to grow grains is just the same as for other garden crops. Just make sure the seed is well-covered and the soil has no big clods. Most grains do not want vast amounts of fertility, because the plants would get too tall and leafy. That can cause the plants to lodge which means to fall over, ruining the grain.
Farmers often use winter wheat, rye, or triticale to "mop up" the nitrogen that is still in the soil at the end of the growing season, so that it isn't lost to winter rains. The nutrients that would otherwise run off are safe in the leaves of the plants, to be released in the compost pile after the grain is cut and threshed.
For spring and summer-planted grains, growing a legume cover crop over the winter (ex. vetch) should get the soil ready. Extra-tall or leafy crops are the exception? Corn, amaranth, and quinoa will want more nutrients, like a nice topdressing of compost, before planting.
A short lesson in Grain Vocabulary