From the parking at Le Laus follow the road south (towards the Col d’Izoard) until it is blocked off. On the left, there should be a ski de fond track which you follow into the valley on the left (south-east). From the hut after a long straight section it bears left and climbs steadily. At around 2052m you should find a small bridge on the right of the track. Take it and then work your way south- west through the trees. As the trees thin out you will see L’Arpelin east face on your right. Continue going south into a small bowl. As the ground starts to rise towards the Cote Belle turn right (west) and make your way to the Col Perdu.
Head back the way you came, it is possible to stay on the left-hand bank of the river you crossed for longer and then cross back over to the ski du fond track further down.