Our house came with a generator hookup already installed by a previous owner. How do I use it? Presumably I attach the generator to the 4-prong socket outside, and get it going, but what sequence of operations is necessary at the panel (see photo), inside? Presumably I first need to disconnect the house from the electric grid by throwing the main breaker. Then how do I know when the power outage is over and I no longer need to use the generator?
Each switch has 3 positions, as labeled on the right hand side of the panel: GEN OFF and LINE. In their current position, the normal electrical feed is going to each circuit. If you move the switch to 'GEN' it will then take power from the generator. OFF lets you kill the circuit entirely (connects to neither line nor generator).
If the panel was properly installed there should be no need to kill the breaker at the main panel: Flipping the switch on this panel disconnects the given circuit from the main panel.
With this type of panel, you do not need to disconnect the house via the main breaker. It's OK to leave the main breaker on, and the rest of the house on utility side - however there will be surges when the power comes back. This type switches over each circuit independently, which is a feature I don't ever see needing. Why would you have some on utility and some on generator? The upside is it makes it easy to bring the load onto the generator gradually.
You'll know when power comes back on because all the other loads in your house will come back on - the ones that are not on these six circuits.
If in the future you find "only six circuits" to be stifling, don't go to an 8- or 10-circuit version of one of these things. There's another way that costs 1/3 the price, has better quality tier, greatly simplified wiring, and lets you support any number of circuits. It also plays well with GFCI and AFCI breakers, which this board will not.