Step 1: Select line items > Selected Items drop down > Set Markup
As shown below we have selected two line items within an invoice and gone to Selected Items and Set Markup.
Step 2: Enter the markup percent and OK
Step 3: The markup has been set
To view the markup select the tag in the grey bar as shown below. Only you will be able to view the markup, to make sure your customer can't see the markup go into options and columns to hide, see the article; How do the printing options work in Invoices and Quotes.