If you have cracks in the “inside” corners of walls you have an easier and a touch more logical solution. Instead of using mud or spackle you can use regular painters caulk and then finger finish the corner. The caulk is paintable and when you use a finger it finishes smooth. Then touch up with paint and your done.

If you use mud you may have to apply and sand and possibly apply more compound and sand again. Just be sure to use a wet finger. Been at this stuff a while and this has been the best “trick” yet.

Have fun and good luck.