If you are serious about breeding for New Hampshires, the simplest thing to do is get a trio from a breeder to start your flock. Otherwise, it will take a lot of time and a lot of culling.
Having said that, you can start with reasonably good types from the hatchery, or at least the best you have, for hens, and then purchase a good rooster from a breeder. Usually roosters can be had for cheap as they are over abundant. If you have half way decent hens, that is not a bad way to go as you can line breed back to that rooster for several generations, culling those that do not meet standard. By the third and fourth generations, you need to breed siblings from different generations together, and then hopefully add another genetic line from a good breeder to prevent inbred problems.
The other option is to take your current rooster of choice and several of your current best hens for New Hampshire type then confine them to a breeding pen if you have more than one rooster running lose (which I think you do). If you only have the one preferred rooster, they all can run free, but you have to select hatching eggs from only the preferred hens. It is essential preferred hens have access only to the preferred rooster or your goals will never be successfully achieved.
It will be impossible if you have a completely mixed flock with multiple roosters running indiscriminately. You will continue to infuse the gene pool with too many divergent genes. You can breed for egg production or meat production, or a balance of both, and sustainability of the flock, eventually creating a mutt mixture that is hardy and prolific, but you won't advance a standard...at least not easily in your life time.
The concept of breed is the ability to breed forward like qualities in the offspring. When you have divergent birds, and breeds, running around together, you get a hodge podge result with exponential genetic possibilities.
Not necessarily bad if you simply want to breed a sustainable hybrid flock, strengthened by diversity, choosing the qualities you desire (meat or eggs), but not a good choice for breeding towards a particular type.
A fun read in breed development (and the problems associated along the way) is Nettie Metcalf and her development of the Buckeye breed.
http://americanbuckeyeclub.blogspot.com/p/breed-history.html
Good luck in your endeavors. The New Hampshire is a wonderful bird, and it is sad that its breed has fallen into disrepair, being overtaken by the production hybrids. It would be great to see more dedicated New Hampshire breeders. (I actually have thought to do that myself finding the New Hampshire preferable to the Buckeye experiment I did awhile back,)
LofMc
