Depends on the size, the thickness and the material they are made of. Up to about 30cm square and not porcelain, you will probably find you can get good results on straight cuts with a combined scoring/snapping device. Wickes sell quite a good one for about �20. Any bigger or for porcelain tiles you really need a water grinding wheel. Probably about �30 for a simple and cheap one if you've only a few to do. I use a better quality �70 one.