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.
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