Mix the powdered gap filler in a bucket with some water until it's the consistency of toothpaste. Using your paint scraper, apply it to all of the cracks and chips on the wall, making sure to overfill them.
Use a multi-purpose gap sealer and a caulking gun to fill the gaps in the corners of the walls. After applying the gap sealer, use a damp cloth to wipe off any excess. This will also make it smooth and push the filler it into the corners. You may need a second layer of gap filler to finish off.
When your gap filler is dry, use sandpaper on a hand sander to get a smooth finish. Remember to wear a dust mask when you're sanding. Wipe away any dust and your walls are ready to undercoat or paint.