STEP 1 DOWNLOAD THE FREE WP ALL EXPORT PLUGIN
https://en-gb.wordpress.org/plugins/wp-all-export
STEP 2 CREATE A NEW EXPORT IN THE WP ALL EXPORT PLUGIN
Once your plugin is installed, in All Export in your menu, click on "New Export"
STEP 3 SELECT POSTS
In "Specific Post Type", select "Posts" (as this is what PowerImporter will be migrating).
STEP 4 CUSTOMISE EXPORT FILE
Click on "Customise Export file"
Then click on "Add All" (this is recommended so you are sure your file won't be missing any fields)
STEP 5: CHOOSE YOUR EXPORT TYPE
Click on Feed > Simple XML Feed
STEP 6 : CONFIRM AND RUN EXPORT
Click on "Confirm and Run Export"
STEP 7 : DOWLOAD YOUR FILE
Click on the XML button to download onto your computer.
STEP 8 : UPLOAD YOUR FILE IN POWERIMPORTER
In PowerImporter, step 2, upload your XML file.
See detailed instructions here.