Diet is also very important; I feed raw lamb alongside a complete greyhound racer food. (fed separately). The first 6-7 months of a puppy's life should be devoted to training.  I never allow my dogs to chase rabbits until they are actually ready. I have seen so many dogs ruined by working them far too young, however I have yet to see a single dog ruined starting it later. Every dog is different and some may well be ready for light work much earlier than others.  I usually think about entering a dog when it's around 10 months old. Even at this age they only want very little work and please make sure the dog is fit.
Vixen retrieving the dummy
Fitness is very important in a working dog, especially in a young dog. If you try entering a young unfit dog to work the dog could become frustrated and start to yap at the rabbit it's chasing out of shear frustration. I make sure the dog is ready by road walking the dog fit. At 10 month old my dogs get a 2 mile brisk road walk in the morning and a 2 mile brisk road walk in the evening along with ½ hour free running (on it's own). You actually want the dog trotting when walking it, a slow walk is no use at all when you’re trying to get the dog fit neither is letting it chase around with other dogs. One to one is best as in my experience the dog always looks up to you and it responds to commands much better whilst out in the field for the first time.
Some people like to enter their dogs alongside experienced dogs, I prefer to enter young dogs alone as it encourages them to think for themselves.  If you enter a young dog with an experienced dog the youngster tends to rely on the older dog and they tend to follow the older dog around.
I tend to start my young dogs ferreting first. This is done by entering the ferrets into the rabbit warren using them to bolt the rabbits into the nets or just for the dogs to chase. I tend to use the ferrets to bolt the rabbits into purse nets and long nets for the first few occasions. It gives
the pup time to realize what is actually going on. We forget how much information a pup has to take in when ferreting for the first time. It must be mind blowing for a young dog, and some will take to ferreting much quicker than others. I like to train my dogs to hold rabbits which have been caught in the nets as I do a lot of ferreting alone. A whippet can get to the caught rabbit much quicker than you. As long as they are not hard mouthed no harm will be