1) Register for a user account on isismapping.org.
2) Login.
3) Open up a blog post.
4) Upload your .kml file (.kmz's will not work) to your web space. If you would like to upload to isismapping.org (no FTP required), click on "KML Cabinet" in the "SITE TOOLS" menu on the left. Click on the "KML" folder and then click "File Upload" in the bottom left. Browse for and upload your file.
5) Go to Google Maps and type the url of your file in the box at the top and press "Search Maps". If you uploaded the file to the KML Cabinet, the url will be: http://www.isismapping.org/sites/default/files/data/KML/yourfile.kml , with "yourfile" replaced by your own file name of course.
6) Click the "Link" button on the top right.
7) Copy the embed code into your blog post.