Chrome Extension from Scratch to Operational

I have a super quick chrome extension I want to make. I want the chrome extension to give an alert when on certain websites. I will have a list of websites, I want to be able to update the list, and when a person is on one of those websites there is an alert that pops up. For instance, if I was on [login to view URL] or [login to view URL] then I want an alert that pops up with a message. The alert does not need to be a page alert but something just at the top like a flag. So I have a personal self updating, lets say, google sheet or excel sheet, that has a list of websites, and each website could have a message that corresponds to what alert i want to pop up for that site. If a person is on a site like [login to view URL] and goes to [login to view URL] another flag should not be shown because that would be annoying. Also it doesnt have to be a flag it could be like the tab is colored red or something to show the website is on the list. If possible the flag should display just like a quick one-two sentence message but if possible there should be the option to expand and read a larger message that may be a few more sentences. So maybe I have a google sheet or excel sheet with a list of companies, a short message, and a longer optional message. But the core feature is just when on a certain website there is an indication that you are on a website that is on a list I have. The other stuff mentioned about messages and words might be extra.