Provide good living conditions, and they will clean themselves up (filthy feet, etc.)
Provide clean water and as much chick starter* as they want, and they should gain weight until they reach what is healthy for them. For any age, you can also sit out a dish of grit. If they are females anywhere close to egg-laying age, you should also sit out a dish of oyster shells, so they can eat it when they need calcium to make eggshells.
Check them over for mites or lice, and consider getting their droppings tested for worms. If you find any of those, get the right treatment and use it according to the directions. (Different parasites require different treatments, which is why you need to identify what they may have before you can treat. And if there are no parasites, you do not need to treat for any.)
*Chick raiser is not the only suitable food, but it is usually easy to find and buy. Flock Raiser, or All-Flock should work equally well, and there are probably other brand-specific names for suitable foods too. You want something that provides everything a chick needs to grow, because that is also suitable for a chicken of any age that needs to gain weight or has been malnourished.
