Making index in Thot is explained in the section 22 of the Thot Manual
(Thot/doc/thoteditor/Thotman-E.PIV)
In a few word you have to :
1- Activate the entry "Index to build..." from tools menu
2 - This let you insert index marks in the document by selecting the word you
want to index and activate the menu entry Edit/Insert (or click on the left
button in the document sheet) and choose Index_mark around.
Once you've inserted th first mark, a view called "Descriptors" is opened.
With this view you can check the indexed words and link new marks with an
existing descriptor.
3 - Once the marks are created you can build the formatted view of the index
with the entry Tools/Index to build. The final view is then showed (in blue
because it is a read-only view).
Hope this helps.
Stephane.