Socialization is critical. A kitchen or family room is the ideal spot for a play pen. Exposure to daily noise and activity is half the job done, and ensures that youngsters are available to be played with whenever there's a spare moment. I don't do any leash work with very young puppies. They learn to follow my voice and movements while affording me a good opportunity to spot timid or independent individuals.
Trotters tend to be better movers as adults.
Later, leash training is done with a soft, non-slip show lead with calling, and a little tug and release when needed. I personally avoid using food when leash training. We are training dogs to look forward as they move out, not up at one's hands and face. Leading puppies around with goodies can instill awkward movement habits. Train at a distance from the house and littermates and keep the sessions short.