How remarketing can be done effectively?

There are many such guides so take your pick. They should be at your local bookstore, maybe even the library and of course online. There's many general marketing books that can help too.

As for remarketing, I think most use it in a wrong way. It depends on many factors but most that I've seen simply use the same ad used to attract me to their site in the first place. You don't want to advertise to me the second time the same way you did the first. You know I've been on your site so do something different, remind me that I was there and why I should come back.

For example, if you sell air conditioners, and I happened to find your site (in any way, organically or search ads), if I haven't bought from you, you would want to remarket to me, maybe saying "Didn't buy that AC yet? Getting too hot?" and remind me to get that AC I saw at your store. You don't want to use the same ad you did when I first searched for you.

Not every thing is suitable for remarketing. Any product or service where customers need it now (a doctor for example), a remarketing campaign probably would not work very well. Or take a funeral director. Usually, we look for these only when we need it and we need it now. Once we use the service, we don't need it again until the next time. So remarketing coffins or funeral services is not likely to make people say "oh yeah, I forgot about them, let's go back to their site and see what's new".