i mixed it right into the soil. about 1/4-1/3 casting to 2/3-3/4 soil is pretty efective. also, about a 40-50% mix in the hole where a transplant is being relocated is a healthy boost to the plant. (as well as a little molassas)
Put the castings on top of the soil. This way when you water all of the nutrients flow down to the roots.
^ I usually mix it for the initial soil, and when the soil settles during watering, i put a richer mixture to re-fill the container