For the 5-5-5-5 diagram, use standard re-entrant tuning (g C E A). From string 4 to string 1, this shape sounds C, F, A and D: all four F6 chord tones (fifth, root, third and sixth).
Lay your index finger across all four strings at fret 5, just behind the fret wire. Pick each string separately and adjust the barre until every note rings; use only enough pressure to keep the notes clear.
Strum all four strings gently, then pick the A string on its own to hear the D that gives F6 its sixth. If one note buzzes, check it separately before returning to a slow four-string strum.