It's easy -- most dogs learn in 24 hours and it doesn't harm them to go even a day or two without food unless they are sickly. People regularly have to do this with cavalier puppies who very very often become fussy eaters!!
The general rule is: put food down for 10-15 minutes, without making a big deal about it or encouraging the dog in any way. After 10-15 minutes, lift the dish without any comment or fussing, and put away until the next scheduled eating time. Dont feed any treats between meals (a few little training treats are fine).
They learn very quickly. I have had all sorts of fussy dogs and rescues come into the house that people told me had to have their food crushed, be hand fed, be fed on plates, be spoon fed :yikes. What a surprise to find that the above method has dogs eating normally within days... :lol:. Having a couple of other dogs happy to eat the ignored food also helps fussy dogs convert to normal eaters, too!
I actually have been going through exactly such a transition with my formerly free-feeding cats. One had become chubby and another heading that way. Two others, I wanted to put on a raw diet. So now everyone is on raw and fed in the morning and that is it. For one (the chubby boy) it took a couple of days of picking briefly at the raw food, which he didn't much like. three or four days later, he eats all his raw food in the morning.
Switching cats is I think probably a lot more difficult than switching a dog -- they are a lot more fixed in feeding routines and free-feeding.
I do know people who leave food down during the day and it can work for a solo dog but generally it is easier, more hygienic and more useful in many different ways to have a dog that eats within a timeframe.
We had a cavalier that came into rescue (eventually -- and unfortunately -- reclaimed by a really stupid owner) who was skeletal thin and matted... given to scavenging on the estate... vet was quite alarmed at how thin he was. Turned out the owners were freefeeding him unwatched in the back garden with their husky who was clearly eating ALL the food. Idiots didn't even notice their dog was in a dire state -- clearly a dog dumped all day and night in the garden and hardly cared for. Wish the woman who found him had never put up posters in the area as that dog sure needed a better home.