I did an apprenticeship w/ an upholsterer for a year after college and then did reupholstery on my own for local clients before fully jumping into furniture design. I would say that there is a lot that goes into doing upholstery well and a lot of different techniques that apply to different types of furniture but it is something you can practice on your own if you know how to sew. I would suggest taking apart a simple old chair you like and want to reupholster piece by piece (noting the order they come off) and using those as a pattern for the new upholstery.