There is no track or markers up the west branch of Mistake Creek draining U Pass. Enter the trees on the true right and generally keep within earshot of the stream as you climb the hill. There were deer trails around in April 2016 to follow and the bush, despite being full of ferns, was quite open. After an hour or so the bush begins to open up into scrub so drop down to the stream bank of the west branch and rock hop up the stream for best travel. The daunting waterfall tumbling out of the upper hanging valley points the way.
The following is the way my party went and I went up, it is not necessarily the best way or the way Moirs describes. Study the fearsome line of bluffs to the true left of the waterfall. A large fan, overgrown with light scrub lies to the true left of the line of large pale grey bluffs. Climb up the fan where hard under dark grey bluffs a moderate slope climbs up to the northwest. Climb the slope all the way to the top which takes you above the pale grey bluffs and below a pale grey near vertical rock slope.Sidle southwestwards across very steep terrain, clinging to clumps of tussock, then once past the rock slope climb straight up snow grass until under more dark grey bluffs, and sidle around until the gradient eases off on a kind of grassy ledge. Follow the ledge around until a dry creek bed drops down the hill towards the main stream. Cross the dry creek bed and then climb talus slopes up and into the upper hanging valley, skirting the edge of a bouldery area on the north of the valley mouth.
The climb from the top of the slope above the overgrown fan to the ledge just north of the dry creek bed crosses extremely steep and exposed slopes. A slip would be serious. Instead of climbing as high as possible under the rock slope it MIGHT be a better route to only climb as high as the scrub line above the big grey bluffs then sidle across from there, where a slip would be contained by the scrub.
Connect with the main stream bed on the map which in April 2016 was dry (all the water falling over the waterfall emerged from underground below the lip of the hanging valley) and follow this up valley through the tussock to reach an open flat area for camping between the tarn and the stream. Both of these were dry apart from small pools and possibly dry up entirely some years.