The photo below shows Owlet sitting up, unsupported! Yes, my arm is against hers but I promise you she’s balancing all on her own!
Most babies can sit for awhile, without support by the time they’re eight months old. Even babies who are masters of sitting will fall over eventually, because they lose interest in being upright. Laying on the ground is so much more fun! Owlet is waaaaaaay ahead with this milestone she is almost 6 months and is already sitting up on her own. She’ll stay up for about a minute at a time before she’ll loose her balance or just topple over because she’s bored with that particular position.
So how can you help your baby learn to sit up? You could always try to prop your baby in a sitting position from day one but true independent sitting doesn’t begin until she has head control. On average at about four months your baby’s neck and head muscles start to strengthen rapidly. She’ll learn to raise and hold her head up, at least a couple seconds at a time, while lying on her stomach.
Up next she’ll figure out how to get herself up onto her arms and hold her chest off the ground in a sort of a baby push-up position. Around five months they may be able to sit momentarily without assistance, though you should stay nearby to provide support and a and soft place to land when they do fall over. You don’t need to surround baby with pillows and cushions unless you’re on hard ground (i.e. tile, hardwood, concrete). Toppling onto carpet won’t hurt them!
Lifting their head and chest helps baby strengthen her neck to develop the head control needed to sit up. You can help by encouraging her to play face down on the floor and then prompting her to look up by getting their attention with a favorite toy or by snapping your fingers. Once your baby is a reasonably confident sitter, put toys and other interesting objects just out of reach, these will hold her attention as he learns to balance with her arms.
If your baby is unable to sit by 8 months, I suggest bringing it up with their pediatrician. Babies develop skills differently, some more quickly and some slower than others, but head control is absolutely essential to sitting independently, and sitting is the key to crawling, standing, and learning to walk. So if something seems off to you it’s best to consult the experts!