It's kind of hard to tell from the pic, but it looks like there's a significant difference in depth of about 1/2 an inch. Once you scrape off any excess gunk, you'll basically be doing what's called a skim coat over the lower area. Plenty of videos around to help. Marshalltown makes something called a squeegee trowel that's beginner-friendly and will really help you get an even surface. You'll just have to sand, prime it, re-spackle any bad spots that appear, and prime again before you paint. Use actual primer instead of the primer+paint.