This is probably the easiest stage of all, but don't rush it. Using Milliput, or whatever material you intend to use instead, we're going to build a rough surface over the wireframes. This will become a base we can then model the final surface over. You could simply model the final surface straight off, but there's nothing solid to really get a hold of at the moment, and adding an underlayer remedies that.
The underlayer really is very crude and simple. Take small, finger-sized knobs of putty and squash them into flat, tile-like shapes. Simply attach these to the wireframe, and embed the wire. The end result will be something that resembles a three-year-old's attempt at making a vase, but all the refinement comes with the next layer.
It's difficult to see in the photographs, but try to keep the wall at the heel deliberately low, as this will later allow you more flexibility in putting the hoof on, and in flexing your hand up and down.
At this stage, trying your hooves on really is critical, since this is the surface against which your fist will rest in the finished article. This is the stage at which to make things comfortable! When you fill in the sole, make sure the wire is covered and no sharp bits of solder are exposed.
NB: I found it easiest to do this stage in two stages: the wall of the hoof, and the sole. Make the sole with a completely flat surface at the moment, with no attempt at realism yet.
To ensure the sole really is flat, take some polythene (any plastic bag is perfect), wet it with water and press the hoof sole-down onto it. In fact, do this with your hand inside it: how comfortable is it? Exposed wireframe or small lumbs might make the hoof very uncomfortable later: make sure all inner surfaces are suitably smooth. While still wet, I pressed my clenched fist into the sole and left an imprint: this later ensured my hand would always fit into the correct position. Weight the hoof down with something heavy - if you did the wall first, the hoof should now be very sturdy and able to take quite a substantial weight. The reason for using the plastic bag is so that the putty does not adhere to your worksurface. It will stick to the plastic instead: when the sole has dried, this should then be easy to peel away.
Once this stage is complete (two days if you follow the suggestion above), you are ready to begin modelling the actual surface of the hoof: stage five.