Codex tools: Log in
Paths tab at the top
New Item icon (second one, looks like a little square)
Keywords tab at the top
New Item icon
__ (that's underscore underscore), press enter
New Item icon
_e (that's underscore e), press enter
Okay
The Update Summary window should display, with a list of strings that were found to translate, based on the keywords supplied above (__ and _e). Click Okay.
Save the .po file, and exit the program.
Make the .po file available for download (or optionally include it in the plugin archive). Translators will use this file to construct a .mo file, which will be used by the load_plugin_textdomain() function.