In STV, instead of voting for a single candidate, you vote for all the candidates you like -- and rank them in your order of preference. The STV voting algorithm (specifically, the Wright STV variant) then attempts to make the best use of your vote to elect your preferred candidates in your order of preference.
STV thus largely avoids the problem of tactical voting. If you can only vote for a single candidate, then you run the risk that your candidate will not get elected -- in which case, your vote is effectively wasted. Let's say you really like Alice, but you think she's unlikely to get elected, and you sort of like Bob, and think he has a better chance. You might decide "If I vote for Alice, the vote will be wasted -- it won't elect her, and it could have been used to elect Bob, so he might not get elected either. So I have to vote for Bob, even though I like Alice a lot more. Sucks to be you, Alice!"
Under STV, on the other hand, your ballot would list Alice as your first choice, and Bob as your second (and as many other candidates as you like as well). When the votes are counted, if Alice doesn't have enough support to get elected, your vote will help Bob get elected. Even better, if Alice does get elected, your vote will be split, and whatever part of it isn't needed to elect Alice will help elect Bob.
This allocation and splitting process continues until there are no candidates left to help, which is why it's a good idea to list all the candidates you like on your ballot. It take a little longer than voting for a single person, but it maximizes your influence.
In the final days before the election, both the candidates and the pundits will be publishing their recommended ballots -- and I will be no exception (though as a pundit this year!). While the candidates obviously want to get elected, they also want to mutually support other candidates they like.
So if you really like a particular candidate, strongly consider using their recommended ballot -- but as a starting point. Feel free to modify it as you see fit.