Here is how I handle resource guarding.
Make a list of the items (or people) your dog guards.
Figure out what motivates your dog to comply. This can be a ball, a toy or a really yummy treat. (Boiled chicken can get a dog to give up most anything.)
Ask yourself some hard questions about your own behavior
Do I pet my dog every time it comes up to me? Does my dog nudge my hand for petting? Does my dog shove a toy into my hand or against me to tell me "play with me right now!"? Do I feed my dog if he goes and picks up his food bowl and drops it or brings it to me? Do I give my dog a treat if he whines at the treat cabinet? If you answered yes to any of those questions, stop that!
Initiate the Premack Principle or "Grandma's Rule"
This is commonly referred to as the Nothing in Life is Free "NILIF" protocol. In other words, no petting, no food, no play, no going for a walk, no going outside, unless they have "earned" it. Now, this is simple. They don't have to jump through burning hoops or take out the trash. A simple, sit-wait or sit-stay, or using whatever "request" you have taught them to go outside will suffice. Working for what they need to survive is natural to a dog and makes sense to them.
Teaching the Cue "Give" or "Drop"
In this section I will be describing a "trade off" method. Start with objects that your dog does not consider "high value." This sets the dog up for success. You must work your way up to the items that are guarded more intensely. You can entice your dog with a treat or another toy to drop the item. (The Motivator.) The second the dog drops the item, pick it up first, and then deliver the motivator and praise lavishly and calmly. Return to your dog the prized item you have asked him to relinquish immediately after he has given it up happily, or finished chewing his treat if that is what you are using.
Repeat this process until you are certain that the dog knows the cue "give" or "drop" and is doing it reliably every, single time, with no protestation. This teaches the dog that giving you the resource is a good thing and often means something even better is coming their way.
Find a dog trainer near your location and take the Canine Good Citizen test for your dog.