I have used a combination of resources to first teach the Hold. If the dog knows what Stay means, it should be easy because a Hold is a Stay for the Mouth....
Now that all said, it depends on your training philosophy, but mine is clear and simple. I show them what I want, and reward that behavior. I generally start w/ a gloved finger in their mouth, and show them what I want. I then progress to other objects, and I do a variety of objects to not make the pup sour on any certain one. Dumbbell is easiest imo. Paint roller, phone remote (yep...), metal articles, a chunk of wood, bumpers, you name it....
My newest pup was Holding beautifully at 3 mos, and had a decent formal retrieve (just to front, no finish) at 17 wks.