Freddie King's approach and demeanor is good.
I also benefited from an article called
A Gentler Force Fetch from Delta Waterfowl. Many nuggets of wisdom in this article.
What helps me and my Boykin spaniel (a dog known for shutting down easy) is using praise and/or
attrition to teach and mild correction to enforce.
Praise didn't work great for me during force fetch because 1) if using food the pup would open her mouth to accept the food and thus drop whatever she was supposed to be holding and 2) verbal praise alone could cause her to smile and feel the verbal praise as "task complete" when the task is not complete until the next step, which is "drop" or "give" or whatever command you choose to get the object.
Attrition was huge -- and when I say "attrition" I mean you keep asking the dog to do the command until there is no option to do anything else so they yield. This strategy must be done with stone cold patience and hard love. No e-collar, no punishment, just keep putting the object in the pup's mouth and holding it there until they
submit. That's attrition. Showing the pup that you are the (gentle) boss and that the best thing they can do to get on with their day is just hold the stinking bumper.
That's the start at least. Timed holds are the next step. I read on a forum once about a guy who'd get his dog to hold and then go in and make a sandwich for 10 minutes and come back and that dog was still sitting their holding a bumper. Sweet.
I started "hold" with a thin piece of pvc with hot dog rubbed all over it. Then moved on to my hand in a leather glove. Then my gloveless hand. Then a weighted pet dumbell that I got off Amazon. Then a bigger one of those. Then FINALLY bumpers (which she loves of course), bird wings, and frozen / live birds in that order. Two major concepts here: you introduce things that they have no instinctual interest in first (the dowells) to teach the concept of hold and then once they understand "hold" as a command you build up to birds. At first, pup will want to chew and plan and run away with the bird -- if you have a birdy dog anyway.
Last comment, your hand in glove and finally bare hand is an AWESOME tactic for keeping pup from chewing on birds. If they chew on your hand, you scream "OW!" and pup likes you, so she doesn't want to hurt you and won't bite. Later, when pup chews on a bumpeer or bird, you scream "OW!" and pup knows, okay, that hurts and I'm not supposed to chew. I hate hunting with mouthy retrievers. Don't let your pup be a bird eater.