I want to help to translate some post and i don't know how to use the notes

On the lower-left of a post, you will find the Options header and under it you'd find Add Note. Once you click on it, a blank note will appear on the image. Now you are able to drag the blank note around. You can also squash and stretch it to fit wherever you are making the note by dragging the small dark box on the bottom-right of the note. By clicking on the box that appears when you hover over a note, you can edit the contents of the note, see the history of it, remove it, and save it.

Also be mindful that you are making the note while the image is at its original size and is not resized. If you save notes on a resized image, the actual location of the note will not be where you desired it to be