Here is the steps of using poEdit:
- Download and install poEdit
- Download the official WordPress POT file
- Open the file in poEdit.
- The upper-left box is the original message (in English) from the POT file. The lower-right box is where you add your translation.
- Go to File → Save as… to save your translations in a PO file.
- When you are finished translating, go to File → Save as… again to generate the MO file.
Please read on the original document and here is a better explanation in Chinese.