Inner Mast Stay ??
It is not a simple task to add a babystay. A jibstay is even a bit worse, Beyond that, a babystay frankly won''t keep your mast up in most dismasting situations and besides a babystay makes it much harder to tack as you have to drag the sail over the stay on each tack. Typically, when a forestay breaks the forward lower shrouds and luff wire in jib are usually enough to keep your mast up until you can rig the spinacker or spare jib halyard as a temporary forestay. In my experience you typically loose a mast when an upper or lower shroud fails.
To add a babystaym you need to glass a sting back and transverse frame to the centerline of the boat in the interior of the hull in a direct line with the stay.
At the deck you will need to through bolt a heavy duty eye above and below the deck. These eyes will need to be in the line of the Babystay. To install the eyes, you need to predrill the bolt holes through the top skin of the deck only being careful not to pierce the bottom skin of the deck. Then you need to chew up the core in that area. The standard method for doing that is to take an allen wrench and cut the short leg about 3/8" long and chuck it in a drill. Then stick in into the core of the deck and let it spin until the core has been chewed up. Then vacuum the chewed up coring out using a shopvac. Once the hole is cleaned out, mask the hole, mix up a little epoxy and partially fill the hole to wet out the core in the area and then fill the hole level with the deck with epoxy thickened to the consistancy of peanut butter. When the epoxy has set redrill the holes this time all the way through the deck.
Tape off the foot print of the two eyes (one above and ond below) and then apply caulk to the upper pad eye and thru bolt through the deck and the eyes above and below the deck.
Once the caulk has set, below deck, run a short length of rod rigging with its own turnbuckle from the new strongback/transverse frame to the pad eye below the deck and tension it lightly.
Depending on the size of the boat and the constuction of the mast, either bolt on a ''Y'' tang at the correct position on the mast or have a tang welded on. Finally run your new babystay from the tang on the mast to the pad eye and tension it and the rod below the deck at the same time.
You will probably want to add a shroud roller to reduce the tendancy for the jib to hang up but it still will. Of course if you are adding a babystay and planning to really load it up then you should probably add running backstays as well to offset the forward loading at an unopposed panel point.
Now then, Just out of curiousity what kind of boat are you thinking of doing this to?
In my book adding a Babystay makes virtually no sense on a boat under about 40 feet and even then it is questionable.