Crooked walls and ceilings can make it challenging to install crown molding. Any attempt pull the molding into contact with the irregular surfaces will usually result in trim that looks twisted.
One way around this problem is to fashion small diagonal blocks and attach to molding to them.
Using a stud finder locate and mark wall studs. Although walls typically have boards called top plates running horizontally at the top of the wall, nailing the blocks at the stud locations will provide additional nailing surface – something that could be advantageous for larger crowns.
Attach the corner block by applying construction adhesive to the backs then nailing them in place. I strongly recommend using a brad nailer for this job -- it's so much easier and faster than a hammer and nail and avoids any wall damage from pounding or missed hammer blows. Hand nailing also runs the risk of splitting the blocks.
Another alternative is to pre-drill the blocks using a 3/16” bit and install them using drywall screws. Using ¾” plywood for the blocks minimizes the likelihood of splitting the wood.
Once the blocks are in place, nail the crown molding to the diagonal face and caulk both the ceiling and wall joints.