Approach

From the Konkordia Hut
Traverse the immense Konkordiaplatz, taking care with the glacier streams that can be difficult to cross towards the end of the season, making long detours necessary.
Go up the Jungfraufirn  trending progressively to the true right bank, until about 3400m, under a snow and rock spur which descends from the E of the Rottalhorn.
From the Junfraujoch Top Station, (or the Monchsjoch Hut)
In winter and early in the summer:  get onto the glacier glacier and go SE, bypassing the rock and snow spur coming down from the E of the Rottalhorn.
Later in the summer: descend the glacier to the SW and before the lowest point of the spur, identify at about 3380m a small snow slope, (or ice) which gives access to the foot of the spur at the level of a rain gauge.  

Route

To the Rottelsattel
In winter and early in the summer: climb a short S facing slope at about 30° to gain a large snow hump, (Pte  3506m).

Later in the summer: Go up to the rain meter.  Climb up to the sign just above.  Then follow to the NW an obvious small path up the crestwhich leads to a step which is climbed at 3b.  Two pegs, with rings at the top.  Contour to the W to reach a commemorative plaque.   Go up easy rocks to the large snow hump, (Pte  3506m).

Go without difficulty over the snow hump and traverse a steeper slope, (risk of cornices) to reach the bergschrund beneath the Rottelsattel. (Ski depot.)
Cross the bergschrund, (can be awkward,) and climb the steep slope above, (often icy) to reach the col.  50m rope essential if you want to pitch it.

To the summit
During the ski season, the final slope should not be icy and so it is not necessary to use the rocks on the left, (looking upwards).  Instead go straight up the the brad and magnificent SSE snow ridge  (45°max) to the summit.
In summer however, go up to the foot of the snow ridge then traverse to the left to reach the rocky ridge, (a metal stake in the middle of the traverse.) Several metal fixings in the ridge, spaced between 25 to 50m, protect the way to the summit.

Descent

The same way, or
Variant for skiers (or on foot) by the N bowl of the Rottalsattel :
From the Rottalsatel bergschrund go back about 100 metres to the SE then descend steeply NW between two seracs (S4/200m) to reach the broad gully which is to the N of the spur on the normal route.  Descend this to the E.  At about 3450m go to the true left bank to avoid some crevasses and rejoin the normal route beneath the Jungfraujoch station or go due N to reach the Junfraujoch at Pte 3475.
This variant is not recommended in ascent because it is exposed to serac fall.