HELP! HYDRO KIT ARRIVED. PANICKING!

First of all, I am sorry for posting another thread that people have probably answered a thousand times.
Hello everyone.
I'm so excited. I've been wanting to grow my own for years now and finally have the chance. Yesterday the FloGro 500 kit came, and now ready to start. I'm just panicking about where to start first and what to do .
I've just got the propogation tray out that came with the kit, and have boiled some water, which is currently waiting to be at room temp so I can water the rockwool cubes in the tray before I plant my Grapefruit seeds.
When they have started to germinate, how long should I wait before I move them to the clay pebbles in the hydro tank? And when should I do my first flood and for how long? Also, do I need to flood it once every day, or more than that? SO SORRY, but I'm so new to all this. I don't want to bollox it up!
Sorry guys.

Is this a hydro grow or soil?

soil.

no no, what am I on about. Its hydro. It's got clay pebbles that fill the tank.
Sorry. been smoking.