The editor window main area is a two- or three-column table with a toolbar on top. When you create a new document, you can do one of two things:
Write it from scratch: add new rows for key-string pairs.
Select a reference file and fill the holes. This what you should do if you need to translate an existing file.
If you are writing the document from scratch, use the “+” button to add new key-string pairs. The key is some text used to find the string in the file. The key must be unique in each file; there can't be a duplicate key. You can also add a comment by holding the “+” button until a menu opens (or alternatively you can look in the Table menu).
To edit an existing entry, double-click on the string or key you want to change. You can also reorder strings and comments within the document using drag and drop.
If instead you want to create or edit a document based on the content of another string table document, use the reference button to select that document. Setting a reference file will change the view so that strings from the reference document are on the right and strings from the currently open document are on the left (optionally, you can also make a keys visible in a third column that will appear before the two others).