Acid wash & neutralize the existing plaster.
Cut out the crack in a V shape (cross section view) and fill with hydraulic cement.
Remove the light to the deck, remove all fittings.
Chip away the plaster around the light, all fittings, steps. If you're replacing all tile, remove it.
Roll out SGM Bondcote.
Do any facing work necessary for setting the tile, then set. It should stick out a little past the bond coat. If not, you'll need to chip away under the tile. The removal of old plast around fittings & such is so that the new plaster can be trowelled flush.
Grout the tile.
You're ready for plaster. I would really, really recommend hiring this out. You should be able to find a pool plaster company that will bid the job based on you doing all the prepwork. You doing the prep work should save you roughly 1/3 of a full bid. If you are planning on doing it by yourself, I'll have to add another "really" to the 2nd sentence of this paragragh. Even on a small pool (~85') we have a 6 man crew working it, 4 of which are trowellers. The plaster has to go down all at once (seamless) and will fail prematurely if overtrowelled or trowelled when too dry.
There are nuances to each job so I know there is something on yours I haven't addressed. Let me know if you have any more questions.