If you only want to see one related record at a time, you may not need a portal at all.
But this could be a one row portal with a scroll bar where you click the scroll bar to move forward and backwards through the portal rows.
Or you could use two relationships, one that matches to all records that you want to see one at a time and a second relationship that matches to just one record. Forward and back buttons can update a global variable and then use GetNthRecord to access the "nth" related record in the first relationship to set a match field in your layout's table to the Unique ID of that Nth record to show that single record via the second relationship. This requires two table occurrences of the same related table in Manage | Database | Relationships. A portal is not needed for this method as you can put fields from the table occurrence used in the second relationship directly on your layout.