If it's dried out good and proper, the only way to do it properly is to give it a good few applications of
pure neatsfoot oil, be sure you haven't bought neatsfoot compound. Put it on with a sponge from the flesh side only, not too much at one time, and work it in with your hands. Leave a good while for it to soat right in, and repeat as neccessary; the working and massage of the flesh side of the leather is as important as the oil as this is what will make it regain it's suppleness. You can use a little very gentle heat whilst flexing the leather.
All the waxes and lotions and balms and polishes are for after you've got it back as well as you can, pure neatsfoot and massaging the leather are what'll bring it back to life. Hope this helps