How to help baby sit up?

When I lay baby down flat she tries really hard to sit up. She lifts her head and shoulders but can’t quite figure out how to do it. Any ideas on helping strengthen those muscles?

I know there’s floor seats and stuff but her Physical Therapist made it very clear that we should only use those for a very short time everyday if we need to take a shower or something and that she needs to be laying on the floor on her back or during tummy time most of her awake time.