Being a developer is about looking at the problem and figuring it out. I've posted above the 2 main ways to do this. But you could just go look at the website and figure out that out. View the pages, monitor the HTTP traffic, you would see what options are available for "watching" a page without overscraping and getting blocked banned. That is the easy part, the analysis, which you should know.
One you know what has to be done (your algorithm):
* Now automate perform the scrape to get the new tweet
* Now automate the login with the account
* Now automate posting the reply.
If you are asking for advice on how to know what to do, actually doing it, is going to be much harder. But maybe I am wrong and you were just too lazy to look to do your analysis, but then if so, you will be too lazy to do the work
As I said originally, respecfully, if you say then you are not aware of the complications in actually doing so and the maintenence it requires to keep a bot up to date with a site like twitter