Warped cabinet doors don't close very well and don't look very good either. Older cabinets are more prone to warping than newer ones, and generally cabinet doors may tend to warp more if they are poorly finished or in a high humidity environment, like near kitchen sinks or in bathrooms. There are two main types of warping: centerline warping, which is a bow in the wood running either length wise or width wise, and a twisted warp in which opposite corners twist away from each other. Each needs to be dealt with in their own way.
Remove all the hardware from your cabinet with your drill driver and the correct bit.
Position patio bricks on the bench that will hold up your cabinet door a few inches above the bench. They will be placed running parallel to the warp, as in a length wise warp will have patio bricks supporting the door on the long ends, and a width wise warp will have patio blocks supporting the door on the short ends.
Place the cabinet door onto the bricks with the bow facing upwards.
Place the board directly over the warped bow.
Gently place the concrete blocks on top of the board, and you will notice that the bow will begin to even out from the weight of the blocks. Ideally, you want to put on enough weight to slightly warp the door in the opposite direction because when you do that, the springy nature of wood will return the door to even when you remove the blocks.
Allow the weight and the humidity of the room to unwarp your cabinet door. Check this every other day or so, but it may take a week or two to permanently unwarp the door. Be patient.
When the door has unwarped, take the blocks off and return the door to the room where it will be mounted. Once there, place it on the floor or another flat surface and put the blocks back on top just to keep it flat and level. Let it dry out and acclimate for a couple of days before remounting.
Measure your cabinet door from corner to corner. Use these measurements to purchase the correct threaded truss rod.
Screw in the top of the truss to one of the twisted corners. Use a drill bit for a pilot hole and tighten down the 1/4-inch screws.
Attach the bottom mount in the same way on the other twisted corner.
Begin to hand tighten the threaded adjuster in the centre of the truss. This will pull the two twisted sides together and once again level out the cabinet door. You may need to use a small adjustable wrench to help tighten the threaded adjuster.
In most cases, and because it will remain unseen, you can actually leave the truss in place for years to come. Doing this will assure you that the door will never warp again.
You can only screw trusses like this into hardwood or plywood. Any wood composite you try this with will result in the screws being pulled out as you begin to tighten the truss up.