No need to glue down separate 'tiles' Rob, although that is a better way for realism. Once the sheet of card is glued down and set, the shapes are marked out in pencil, using a ruler (squares or rectangles), the scored with a craft knife, removing any card 'swarf'. The tar sealer, if required, can be done with a pen, although I normally used thinned paint, painting in some areas of 'spillage' or where the tar has flown over a corner joint.
Making the base is very simple Rob, the painting is the thing which adds the realism, and a little practice will allow a great base to be made. Always start off with a shade lighter than required, and allow for some absorbtion by the card. Successive coats, uneven spraying, and multiple coats in some areas add the varied tones, with stains being done with thinned paint.