Cut the block into 3 or 4 chunks. Knead the clay so that it's soft. Lube the car and rub the clay over the wet area. If the clay drags then use more lube. Wipe the panel and the paint should be smooth. Bilt Hamber do Auto Clay which can be used with water as a lube.
Once the clay is comtaminated, knead it again so that there a clean part. If you dop it, throw it away (which is why you don't use the whole block).
Remember to clay the car after washing it.