Tiling is done much like the real thing. First add a very narrow strip of card (say 2mm wide) all around the roof bases. Next add all the lead flashing around the chimneys and wall joins.
Draw some parallel lines on the roofs with a pencil to act as a guide (some of the kits have this etched on).
Use neat PVA brushed on or a spray-mount from a craft shop. Add the rows of slates, trimming roughly now and finally when all is dry. Weather the slates if required.