If you are using an old fashioned grinder, use the medium-sized grinder plate and set the grinder on the edge of a table with a large bowl or pan to catch the fruit mix as it is ground. Old fashioned grinders tend to leak juice down the grinder base, so you may want to set up a bowl underneath to catch the drips.
If you don't have an old fashioned grinder, you can use the grinder attachment to a KitchenAid mixer, or you can use a food processor. If you use a food processor, be careful not to over-pulse! Or you'll have mush instead of relish.
You can also just chop very finely by hand, but that's difficult, especially with the cranberries.