What is the best protocol to use for focus formation assay using MEF cells?

I want to do a focus formation assay using mouse embryonic fibroblast which I knockdown for a gene we think is a tumor-suppressor. Should I coat the petri with something? What is the number of cells to plate when using a 6-well plate? How long should I wait before staining with crystal violet 2-3 weeks? More? I have looked at and tried different protocols but so far nothing really seems to make sense. Thanks for your help.