I began cake decorating using a sharp knife to tidy the base of my cakes but it came with the risk of creating unwanted incisions in the fondant and board.
Then I discovered this PME cutting wheel which achieves the tidiest of finishes and is a breeze to use. It is pictured here on blue marbled fondant for a fisherman's cake (to simulate water) that I made. You can see the precision lines it cuts when rolled through fondant icing. It is most commonly used in petal and leaf construction or for freehand shapes.
We also stock a fondant trimmer wheel which is great for those bigger cutting jobs.