You start with the bottom edge, work your way up, knit front, back and sleeves separately and will eventually find yourself having to sew all the pieces together. Only then can you actually try on your jumper.
What if there is a different way of doing things?
Top down knitting is becoming more popular, and for good reason: you can try your project on as you go and adjust the size. You can knit your whole jumper in one piece - no sewing required! All you need to do is change your mindset on how to do things.