1. Lay out the bark on the roof - this is another layer of damp goodness to stop the soil drying out too much (when buying the bark make sure its not chemically treated to stop weeds etc, as this will not be good for the plants!
2. Once you've got a good covering of bark start to put the compost up. Easiest way I found was to put it into 5 piles then gently (making sure not to move the bark etc) rake it around
3. Do the same thing with the top soil
4. Give it a gently bit of turning to mix it up a bit
5. Get stomping, you want the soil to be reasonably firm - not fluffy and light
6. Final thing to do is put some paving slabs up - this is for two reasons, 1 - so you can walk around without treading on your plants, 2 - so you can put a ladder on the roof to get to the higher one if needed - otherwise you'll put the ladder on your living roof and may damage the liners etc