If you want to create a note about a marriage event (the actual occasion of marriage) select the Marriage event (or add one if you haven't already got one) in the Facts tab of the Property Box, when viewing record details for either spouse. Then enter the note in the Note field below (you may find it convenient to double-click on the field to open a separate resizable Memo window if it's a long note).
Sometimes, however, you want to enter a note that is not about the marriage event as such, but is about the whole married life of a couple. It makes no sense to add this as a note to just one of them, or to duplicate the note and give them a copy each. Instead you should create a Family Note which is applicable to both of them (and shared between both of them). To do this, open the record for either spouse in the Property Box. It doesn't matter which you use. Then choose the Note tab, and click on the button on the toolbar. Select Add Shared Note for a Couple from the options, and confirm which couple you want it to apply to. A new (empty) note is created and is selected in the list of notes in the top half of the dialog. You can type the note directly into the white box in the bottom half of the dialog (again, you may find it convenient to double-click on this white box to open a separate resizable Memo window if it's a long note).
Although it may appear that you have only entered the note for one of the spouses, in fact you will find if you look at the other spouse in the Property Box that you can view and edit the same note there too (in the Notes tab).