1. Start POEdit
2. Click File -> New Catalog
3. Enter a project name (probably your plugin's file name)
4. Click the Paths tab at the top
5. Click the New Item icon (second one, looks like a little square)
6. Enter the path to the directory containing your plugin file ("." tells POEdit to scan the directory that you will save the file to), press enter
7. Click the Keywords tab at the top
8. Click the New Item icon
9. Enter __ (that's underscore underscore), press enter
10. Click the New Item icon
11. Enter _e (that's underscore e), press enter
12. Click Okay
13. Choose a name for your .po file (probably your plugin's base filename)

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.