Filling cracks in corners of drywall can be hard to do with a putty knife.
An easier method for filling cracks in drywall is to use a pastry bag with an 1/8” diameter tip.
After filling the pastry bag with joint compound, squeeze out all the air.
Put the tip against the crack and squeeze the bag until the joint compound starts to come out. Move the tip slowly along the crack to fill it evenly with joint compound.
It is better to use joint compound than caulk for patching cracks in drywall, since it can be sanded smooth with the surface to make the cracks disappear.