Don't get your knickers in a twist about following the plan to the letter. If you think what the rotation is for then you can work out for yourself what to do. Mainly crop rotation is about reducing the risk of pests and disease, much of which is type specific, and for optimising your use of manures and green manures.
But, specifically, allocate a veg type each year to its own area. This means you could have spring cabbages, followed immediately by curly kale, summer broccoli or radishes, all in the one year. The difficulty comes with crops which go in one year but are harvested the following year, such as winter purple sprouting broccoli. I don't follow a rigid rotation plan, and my garden ground isn't divided into discrete areas, so where overwintering crops grow doesn't bother me, as I simply put that veg type far away from that area the following year.
I would follow the potatoes with leeks, beetroot, lettuce and so on. Leeks are part of the allium family, so perhaps this is against the rules, but if I did that then I wouldn't put onions, garlic or shallots in that same piece of ground the following year. Beetroot, chard, lettuce etc are not part of the major groups so tend to be fitted into odd corners anyway.
The one to be really careful of is not to follow potatoes with outdoor tomatoes, as they are both the same group and susceptible to the dreaded blight.