I have already made a couple sweaters similar to what you said and I just used a simple thumb cast on to do it. I made sure the very first cast on stitch was pretty tight and close so there wouldn't be a whole between the body of the sweater and the sleeve. Don't feel too bad, I have made three sweaters altogether where you cast on the sleeve to the body of the sweater and the first time, I did it the wrong end and merrily knitted away for several rows then looked at it and wondered
! I have become quite adept at frogging!!! I have to do it at least twice per project!