Acidifying and Enriching Soil

What exactly do you add to soil to make it "rich and acidic"?

Well rotted manure added to soil would make it rich. Lots of compost -well cured- would do so too.
To make soil acidic in limestone areas you could add plenty of peat moss or composted oak leaves. If the oak leaves are not acidic enough add some sulphur preparation to it, or some ferrous sulphate. The amounts would depend on how alkaline the soil is and how much clay it contains. Clay tends to resist changes in pH and therefore clay soils need more acid materials to lower the pH.