mr

You likely saved it as an image. If so you use a pen or pencil to write on a printed copy of the timesheet.

Or, you open the image in an image editing app and add text layers to the location in the image where you want to add text.

Or, you retype the timesheet into a spreadsheet app and enter text in cells using a spreadsheet app.

Open the file in Preview and then menu Tools>Annotate>Text.

alternatively select file on desktop press command-p then go to printer and remove timesheet then get pen then fill it out