How to filter WFS data from Geoserver data from table and where to put it?

I have a layer linking to a table "Foo" and I want to filter data from "Foo" when I do Layer Preview. I have seen Filter tags etc. and examples with using it BUT I have no idea where to put the filter code! I have not seen anywhere that tells me what file to edit to put the filter code - tried the featureType and layer.xml and neither worked but complained about the Filter tag... please help!