To list and pick a commentary: 

• Click on a Bible version button (eg. "ESV") 
  then click on the tab "Commentaries"
• OR start typing in the top line

Commentaries include: 

• Quick useful notes, eg:
  Clarke, Family, Geneva, JFB, People, TFG, Wesley
• Detailed exposition, eg: 
   Barnes, Abbott, Keil+D, Lightfoot, 
• Devotional thoughts, eg: 
   Burkitt, Luther, Matthew-H, Spurgeon
• Word studies, eg: 
   Darby, Robertson, Scofield
• Textual notes, eg: 
   
  
  

Much like bible versions commentaries can be viewed in several ways:

• Viewed in a separate panel (as illustrated)
  This means you can navigate through the commentary independently of other Bibles or commentaries you have open
• Viewed in columns in the same panel. 
  This means the commentary will always be next to the equivalent text of any accompanying Bible or commentary.  
• Viewed as interleaved in the same panel.
  
  
  

At present you can't write your own commentary inside STEP. 
But you can put STEP inside your commentary using the webmaster tools. 
Any post can contain links to a STEP page, and any webpage can contain a fully-working STEP Bible in a box. Or you can make STEP appear by clicking on a link. See the simple code here.