To lemmatise manually, with a word list on screen,

 

pull it onto the line you want to join it to.

 

and drop it:

 

You will then see the totals change and the items become visible in the Lemmas column.

 

If there are a lot, you can double-click the Lemmas column to see the details:

 

Page url: http://www.lexically.net/wordsmith/step_by_step/?lemmatising.htm