Engineering a successful high-altitude balloon flight requires attention to a great many details, sorted into several areas. Engineering the ascent system involves specifying the balloon, inflating the balloon (to proper volume), rigging the balloon to the spacecraft, and any associated 'engineers love to tinker' improvements (e.g., an optional flight termination unit or cut-down system). Engineering of the descent system involves design of a proper parachute, rigging, and ensuring the chute is not tangled through all phases of descent. Spacecraft systems engineering is complex, and includes engineering strong, lightweight, insulating capsules, and all interior sensors/systems. The ground systems include site selection, balloon inflation/handling, spacecraft checkout, and mission control.
postscript
Why the label SpaceCat? Many reasons. The design engineer is oft reminded, when wondering whether the current sentient primates will survive despite themselves, of a science fiction book read long ago by Andre Norton, Breed to Come. The project engineer has an abiding love of cats of all kinds, including Margay cats (opposable thumbs! the basis of tool building!) Finally, though rats, dogs, and chimpanzees have gone up in space, there's not been much feline exploration of space (with the exception of the French SpaceCat program, viva la France!). (And no, this Air Force feline weightlessness experiment doesn't count, and seems a bit cruel.)