If you want to insert a memo in the payslip of an employee valid just this month, you have to open the calculation screen of the concerned employee by clicking on the button Compute.
Right-click anywhere on the screen and choose Insert a memo for the payslip:
Type the text. Attention: do not go further than the right margin of an A4 page (there is no automatic return).
The standard payslip accepts up to 4 lines of text. If this is not enough, add more with the format editor.
The memo will be displayed only this month but will be stored in case of reprinting.