When Nathan was a couple months old, we started doing very, very minimal amounts of physical therapy with him. Nathan's therapist, Barb, would teach us "exercises" that we could do with him to help strengthen his arms and legs and help give him a wider range of motion. It mostly consisted of stretching his arms and legs a little bit and doing things like "bicycles," "running," "jumping," and "dancing." Basically, we were just trying to get Nathan's legs and arms used to moving and help him gain more strength.
As he got older and healthier, Barb had started us doing additional forms of therapy. We would place Nathan in a bouncy chair, try to get him to play with things, and practice sitting up. Unfortunately, the bout with hydrocephalus had partially derailed therapy a bit. But once the shunt was in and Nathan was so much more social and active, we began working at therapy again.
Two of our main objectives was helping Nathan get stronger and helping him learn to deal with new positions. Of his 4 months in the hospital, Nathan had spent the vast majority of the time laying on his back. So naturally his muscles were very, very weak.
He had a really rough time holding his head up or even supporting his weight at all. And when we would do leg exercises with him, he wouldn't provide any resistance or push back, although he had recently discovered that he could kick with some proficiency.
Additionally, positions other than laying flat on his back were very, very scary for Nathan. He didn't care to sit up, and tummy time was completely out of the question. He didn't even like to be held up against his mommy's shoulder. He only liked to be held in the standard cradling position.
So we all had a lot to learn and to work on.