This method for making Geshi blocks, using the editor, will still work but there is now a toolbar item for Geshi.

There are two options, one is to create a geshi code block and the other is to create a downloadable snippet. For the first you create a code block and for the second you create a file block. To create a simple code block you open a code block and then add the geshi code word, followed by a colon, and the type of code:

 geshi: php

If you are creating a snippet, you add the file name:

 geshi: php file.php 

See geshi discussion and view the video.