Knit as directed (check the errata - it's voluminous!). However, I had read how everyone had trouble with the sleeves fitting the armholes so I decreased a few less (basically making the larger size sleeve) and it attached to the small size in the sweater without too much tugging.
This was a knitting slog - tons of stockinette. But I love the result. The yarn was not very cuddly in the knitting but is better after washed. I held two strands together and got the perfect dk/light worsted gauge.