Composting Grass and Horse Manure?

Turn it several time a week and keep it moist but not dripping.
It takes oxygen as well as water to get the compost bacteria multiplying quickly. You'll know it is finished when it no longer is as hot when you turn it.

If your grass clippings were green and not dry, this pile is going to be very high in nitrogen sources and lacking in carbon sources. Do you have any fallen leaves, shredded newspaper, wood chips, or other low-nitrogen waste you can add to it? High nitrogen piles tend to be more smelly, if I remember correctly.