Roper is actually pretty specific in this area. I'd follow what he says.
One exception is at the very start. I'm assuming your group will circumnavigate Lake Catherine on its north side, and then those who are climbing Ritter will cross Catherine's outlet and navigate along its west shore on the way to Ritter Lakes?
If so, ignore Roper's advice to follow Catherine's outlet stream to the tarn downstream. That tarn has steep terrain and is slow to navigate around. Instead, continue with your friends till you have reached about the midpoint of Catherine's west shore. Then, part ways and go due west over a little saddle. After descending from that saddle, you can easily cross the stream downstream from/south of the tarn. Continue west and find your way, per Roper, down class 3 ledges to the next-lower tarn fed by the waterfall. Roper's descriptions beyond that second tarn are actually pretty good and specific.
We only found ducks and faint use trails near the bottom of the route, where Roper instructs you to "contour" north instead of dropping further into the deep valley to the west of Mt. Ritter. Look carefully for those use trails - they will help you get onto the easiest ramps for the contour. The further you contour, the more convoluted the route will become as you approach the lower Twin Island Lake - up, down, around, backwards and sideways. Continue to follow faint use trails and a few ducks here and there and you'll wind up at the lake one way or another in some circular fashion.
You will reach the lower (northernmost) Twin Island Lake first. It might be worth simply exploring this lake, especially if you are short on time owing to navigating delays, rather than rushing on to the upper (southernmost) Twin Island Lake. We found the route between the two lakes very slow going.
If you do decide you have time to scramble up to the upper lake, walk due south from the lower lake briefly along easy terrain till you see a very steep chute ascending southbound but a bit left/east of where you might have expected. (To the left of the chute, the terrain immediately drops off to the huge valley below Mt. Ritter.) Once you ascend the chute, scoot around southwesterly till you hit the upper lake. The steep chute is the route we found between the two lakes - not to say it is necessarily the only route.