Map the Email Subject to a header column in your spreadsheet (1).
Add the common data in the cell below the header column and use a formula (2) to pull in custom data from another cell in your spreadsheet. Copy and paste your formula to cells on rows where you wish generate a Mail Merge.
In the case below the formula we will use is
="Training program for "&A2
NB: Only copy formulas to cells where you wish to generate a doc and/or mail merge as the add-on will run on every cell where data is detected.