I'd remove the sod/lawn/grass in the area before doing anything else, then mark out the area of the bed and lay the rock border. I guess you could lay flattened cardboard boxes on top of the grass which would possibly smother it, then add your soil, compost etc. and plants but if you just dump the compost on top of the grass, depending on the type of grass and depth of the bed you could possibly have the grass finding it's way to the surface which would just cause a lot more work. I'd remove the grass in the area first but that's just my way. Hopefully others will pop in with suggestions.
