See the Rich Text Editor discussion for a detailed discussion of creation and revision of formatted text blocks.
For Simple Text Blocks:
Login, navigate to the page you want to work on, click on the pencil tool icon at the upper-right, and select "Work on this Page" or "Work on this Page (All Changes)."
Click on "Click to insert new element".
If there is no such item, then click on the cell properties icon , and select "Insert/Copy Rows..." from the menu that pops out. Confirm that it is creating an empty row, choose whether to create the empty row before or after the element above which the cell properties icon was located, and then click on the "Finish" button.
Observe the Element Gallery; scroll as needed and click on "Text Elements" to expand that part of the list.
Click on "Simple Text Block" to select that choice.
Click on "Click here to define the Simple Text Block element".
Type different things into the header and text block fields of the resulting dialog box.
Click on the "Save" button.
Observe the results.
Submit the changes for publication; because this is a new element, the containing layout table and cell will be modified, so choose "Submit Page"; in class, wait your turn to click on the "OK" button.