Sixt-Fer-à-Cheval is at the end of the road up the Giffre valley — there is no pass out of it, so all routes in are the same routes out.
By car is by far the most practical option. From Geneva (about 75 km) take the A40 motorway towards Chamonix, exit at Cluses, then follow signs to Samoëns and continue up the valley to Sixt. The drive from Geneva takes around an hour and fifteen minutes in normal conditions. Parking at the Cirque du Fer-à-Cheval is pay-and-display in summer and fills up by mid-morning on weekends.
By public transport, take the train to Cluses or Bonneville and connect by bus (line 63) via Samoëns to Sixt. Services are infrequent and the timetable changes seasonally; check with Haute-Savoie's regional transport network before relying on it. In peak summer some shuttle buses run directly from Samoëns to the cirque car park.
From Chamonix, the valley road approach (via Cluses) takes about an hour by car. There is no direct mountain road connecting the two valleys.