On Your "Person/Details" page/screen ...
Go down to the last Section ... that being the "Family Members" (Section)
You should 'see' the "+ Spouse" Button/link, just above your "Name".
Select that "+ Spouse" Button/link; and, go from there.
Here is a "Knowledge Article" in 'FamilySearch':
How do I add a spouse or partner in Family Tree?
I hope this helps.