For translators, Polyglot should be pretty much self-explanatory with only very few things that need explaining.
Before you can start translating, an admin has to grant you the necessary permissions. Please see the Haiku forum post Help translating applications with Polyglot.

The start page with a list of all projects

After logging into Polyglot, the start page shows a list of all the hosted projects. After a project's name you may see a globe symbol, linking the it's website and a sheet icon linking to its bugtracker. Some developers also include the date of the next planned release, so translators can cram to get in their translations in time.

Projects that aren't 100% translated in your preferred language - see Edit profile in your user menu at the top right of the page - are highlighted yellow.

Click on a project to go to its translation page:

The "Translation modules" page of a project, with the status of all translations

Most projects have only one Translation module (one "catkeys" file) per language, i.e. there's only one column with the language progress for the main application. If the software, however, includes for example a Tracker add-on or a library or consists of several applications, you'll see more columns that you'll have to get to 100%.

By default, only languages that are being actively translated are listed. If yours isn't among them, it means that nobody has started a translation yet: Click Show all languages to see it.

If the progress of a translation is below 100% the menu Continue beside the progress bar is highlighted blue and a click brings you to a page with all still unfinished text blocks.
In the menu, the item All text blocks opens that page with all text blocks, the ones already translated and the ones still needing work.
Export lets you download the catkeys of that particular language, just like Export all at the bottom of the list of languages lets you download the catkeys of all languages. The downloaded file can then be edited in a text editor or CatKeysEditor, if you prefer that.

You can only edit text blocks for the languages you have permissions for. All others you can only Export or View all the text blocks.

This is the translation page:

Upload translated catkeys at the top of the "Translation" page

At the top, you can upload a previously exported and edited catkeys file of your language. Be aware: if the developer has updated the original English catkeys of his project, they take precedence. If it has changed - more or fewer text blocks - only the ones of yours that still match will be imported.
So everthing's safe, but you may want to prefer leaving the exporting/uploading to the developer and stick to the Polyglot web interface...

Below the catkeys upload, there's a table with all the GUI texts that need localization. Clicking in a row expands it to the editing mode (last row in this example):

A translation block while editing with a legend of its elements

Yet untouched rows are highlighted yellow and have an active checkmark Needs work. As long as it's set, the text isn't counted as being translated and the percentage status won't increase.
If you are not sure about your translation yet, keep the checkmark set until you are.

On the other hand: Some texts may be the same in the English original and your language; OK buttons are a good example. Don't forget to go into editing mode and unset the checkmark in that case!

Tips

A few points on localization in general, and Haiku specifics in particular: