It is a messy and tedious job; each horse gets between 8 - 12 coats applied with a paint brush. I usually put on 2 or 3 coats in one session, let it dry for a day or so and repeat the process until I have built up the right depth of coat. It depends a bit on the state of the underlying wood exactly how much is needed.
The horse is left to dry thoroughly for 6 - 8 weeks. It will then have some cracks in it and they need to be filled and the whole thing rubbed down carefully before painting. This is a fairly horrid task; it is best done by hand to avoid working right through the gesso and damaging the carving below. It takes a few hours to do it properly and a dust mask is essential to protect your lungs.
Instructions on how to make and apply gesso can be found in Anthony Dew's excellent books. There is a link to his shop, The Rocking Horse Shop, on my Links page.
The top picture shows a horse having its first gesso coat and the next photo is after about 5 coats; the gesso is still wet and shiny. In the other pictures, all the gesso has been applied and allowed to dry thoroughly. The horses are ready for sanding and crack filling.