Start early, because this is a strenuous hike with lots of altitude difference. The views are worth it though. To make it a bit more relaxing, you could stay overnight at the hut.
Start at the car park in Gavarnie. Don’t take the obvious path through the village that everybody is taking, but keep to the right and start ascending. Soon you will reach an amazing view of the cirque and the waterfall. Keep following the track. After crossing a small stream the path starts a steep ascend (bon courage!). Half-way through the ascend, the path starts to be rocky and a bit less obvious. Watch out while you cross some smaller streams on the rocks. It can get slippery. This section of trail offers amazing “ambiance de montagne”, as the French would say. Follow the path until the hut. My guess is you’ll need a small break and some sugar here.
From the hut follow the obvious trail towards La Brèche de Roland. Enjoy the incredible view! After taking in the view, take the same path back towards to hut.
Three trails arrive at the hut. One you used for the approach, one leads towards La Brèche de Roland and the third is the one you need for the descend. Follow this path all the way to the base of the waterfall. Be mindfull while descending, there are a few very steep sections. Some sure footedness is needed. From the base of the waterfall follow to touristic track down, back towards the car park. What a hike!