Most babies learn to sit up independently between 6-8 months. There is a lot that needs to happen before this however. Your baby needs to be coming into good extension with their upper back in tummy time. Rolling should be well under way, back to front and front to back before you try sitting them. They need to develop upper body strength coordination and overall body organization, before sitting is attempted.
Ideally your baby learns how to sit on their own, without you "showing" them, this can happen after they learn to hands and knees crawl in some instances. What is important in babies learning to sit, is that we want them to know how they got into the sitting position, and we want them to know how to get out of it. Otherwise they are "stuck" there.
This is fun for them to be in an upright posture, interacting with us, however, if baby can't get out of the position, they are not learning about their own autonomy. What we are mapping in their body is actually stiffness! Yes, prolonged sitting can cause unnecessary stiffness in their shoulders and hips. Not only that, but when you take that baby who is used to sitting back down to tummy time it will be harder, they will protest!.
If you want them to learn about sitting, I like to practice with babies in what I call a "supported" sit. This is having them between your legs or on your legs if they are younger, and leaning against your body for support. See the picture above. That way they lean their back against you and don't have to stiffen in their joints (primarily their hips and shoulders) to keep themselves upright.
Come learn more about motor development, and have fun in my
Developmental Play With Babies class,