Mender is a review layer on top of a site you already have open. Turn it on, click what is wrong, and say what is wrong with it.
Open a site from Review. Mender appears as a small paper dot in the bottom-right corner of that site; press it to start reviewing, and the dot becomes a panel listing what you can do. Press Stop reviewing when you are finished — the site behaves exactly as it always did while Mender is off.
▁ in the panel's top corner tucks it back down to the dot without leaving review mode, for when it is sitting over the thing you were looking at. Press the dot to bring it back. It stays tucked away as you move around the site, so you can fold it once and carry on reviewing.
The panel opens beside the thing you clicked and can be moved off it: drag the title bar — the dots at its left edge are the handle. ▁ folds it down to that bar and back, and ✕ closes it without sending, which Esc also does.
Every note carries a picture of what you were looking at, and you see it before anything is sent, with whatever the site's rules hide already hidden. Under it: Use the whole element and Crop it myself when the automatic crop caught the wrong thing, Hide more to black out anything else, Draw on it to point at what you mean, and Use a browser screenshot — where the browser offers it — to replace a redrawing with a photograph of the tab.
Or send a picture of your own, which is what you already have if you reached for your screenshot key before reaching for Mender. Drop the file on the panel, paste it, or press Choose a file. It replaces the captured picture rather than joining it, and because Mender did not take it, this site's redaction rules have not been applied to it — the caption says so.
Both have a keyboard path, because a feature you can only reach by dragging is a feature some reviewers do not have. The box is the one ⌘-drag and Crop it myself open; the marks are Draw on it, which opens the picture at the size of the window with a circle, an arrow, a box, a line, a freehand pen, text and a black-out, an Undo, and nothing decided until Done.
Every note on a page has a numbered pin where it was left. Rest the pointer on a pin and it shows what the note says and the picture that came with it; press the pin to open the note in full. Notes on this page on the panel opens the whole list down the left-hand side; hovering a row lights its pin, and pressing one takes you to it. Pressing the small picture on a row opens that picture full size.
The colour of a pin's head is the whole of its status. There is nothing else to learn.
Hide pins on the panel folds them away without turning review off — for the moment in every review when the notes are in the way of the thing being reviewed. Notes you file while they are hidden are still filed.
A note is anchored to the thing it was left on, and a page can change enough that the thing is no longer there. Those notes are not lost: they collect in a small tray in the bottom-left corner, and you can point at where each one belongs now.
Review lists every note you have left, on every site you have been given. Show them as a list or as a grid of their own pictures, in whichever order suits — newest, oldest, whatever has moved most recently, by status or by page — and gather them under a heading for the site, the page, the status or the month. However you set it, the address bar says so: bookmark it, or send it to somebody, and it comes back arranged the same way.
A note's title opens what you sent: the words, the picture, and everything that has been said about it since. On the page, beside it, is the other thing you might want — it opens the site itself with that note waiting on the page it was left on. Replies and does it look fixed? are answered there, on the page, rather than here.
A note is shared with everyone on the site unless you say otherwise. Make private, on the note itself or on its page here, keeps it to you: nobody else sees it on the page, in their inbox or in their mail, and it cannot be queued to be fixed until you share it again. Tick several and the bar under the list does it in one go — the way to hide a batch of test notes, or everything on one site.
Everything above is for telling somebody about their site. When it is Mender in the way — the dot sitting over something, a panel covering the thing you were looking at, a pin landing where you did not click — that cannot be a note on the page: Mender leaves itself out of every picture and every anchor on purpose, so a note about it would have nothing to point at. Something is wrong with Mender is the way to say it instead. It is a page here rather than a panel there, so you can see exactly what is about to be sent before you send it.