Mk1 or MK2 escort?
For MK1, you will need, round headlights, buckets, retainers grill and a sheet of steel/alluminium OR buy prefabricated backing plates OR change the front panel.
Easiest is with the prefab insets, remove original lights and grill, offer up inserts with lights fitted attach grill, check position, remove grill, and weld or rivet inserts in place. Wire up lights, fit grill and you're done.
With blank steel or alloy, weld/rivet in place, offer up the grill, mark out round headlight position, cut out holes, fit up lights, wire and fit grill.
With a new round headlight front panel, drill and grind out the front panel, find additional rust, remove entire front end chasing rust, then offer up new panel, remove and modify new panel so that it fits correctly, clamp and weld in place, paint front end, decide it looks beter then the rest of the car, end up painting whole car....... :)
For a mk2, much easier as the body types didn't change between the two.
Remove the rectangle grill and lights, replace with round headlight buckets, lights and grill. All screw holes and wiring looms should match.
Whichever you do though, it is a good oportunity to offer up some new technology/brighter headlights and add some much needed relays to stop voltage drop and save your switches.
Shaun