1. Is it too late to stain the deck with same shade of green as our home?
No. I recommend a semi-transparent oil on decks.
2. What are the recommended preparation steps before applying the stain?
Make sure the deck is clean and dry. Sanding really depends on the condition of the surface.
3. Anything else I should know such as how many coats of stain to apply? You can apply 2 coats if the first doesn't look right, but I would avoid any more than that as the stain will build up on the surface and be prone to peeling.