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1. download the archive
2. unzip and drop all the files, as is, in your plugins directory.
3. Enable the plugin in the WP Admin >> Plugins section.
4. Browse to WP Admin >> Write >> Posts or WP Admin >> Manage >> Posts then choose a post to modify.
5. P3 is delivered in English and French but can be easily used with other languages. You just have to create a file PpP-xxYY.mo (xxYY being the language code of your WordPress settings. Eg: frFR for French) from the existing file PpP-frFR.po after translation of the text strings located after the "msgstr" tag. The resulting PpP-xx_YY.mo file has to be stored in the plugins directory.

(Note: if you are upgrading from a previous install, simply overwrite the older files with the new ones in the instructions above)