sit next to Nellie
old-fashioned To work alongside a person with a lot of experience so as to learn how best to do a job by watching them work. It used to be the case that new recruits would just sit next to "Nellie" when they joined the team; now, with how quickly technology is advancing, it's often the new recruits who have to explain how things work to the older members of staff.
sit next to Nellie
learn how to do a job or task by watching and copying someone experienced in it. informal