How difficult is it to install a generator transfer switch? It's pre-wired, doesn't look too
Difficult. Is there anything I should look out for? The only thing I'm not sure of is it comes with two 15-amp 1-pole breakers, two 20-amp 1-pole breakers, and one 2-pole 20 amp breaker. How do I tell the difference on my main circuit panel? I mean the amps are
Marked but how do I tell if it's 1 pole or 2 poles?

What they are telling you is that it supports one 15 A 120 V circuit, two 20 amp 120 V circuit and a 20 Amp 240 volt circuit.

In your panel, two pole breakers for 240 V circuits have the handles bridged and each handle is marked.

Electric Ranges, Air conditioners and electric water heaters typically have two pole breakers.

An easier way to do the transfer is with a mechanical generator interlock kit for your panel. This prevents the generator feed breaker from being on when the main breaker is on.

It's a lot simpler, if your panel will accept them.

See: Generator InterLock Kit - Frequently Asked Questions

Manufacturer's of the panels sometimes make them too.