Since supermassive black holes at the center of the galaxy are thought to be the power source for Quasars, the most powerful objects in the visible universe (visible across billions of years of space and time), this isn't going to be something you put together with parts in your backyard or the local hardware store.
The energetic environment is massive (which is why you want to tap it), with the central black hole surrounded by a huge accretion disc (radiating at frequencies from radio to x-ray), and highly charged jets of radiation and particles accelerated to near light speed
All this energy can be collected through means we understand today - the energetic radiation from an accretion disc can be captured by a massive dyson sphere or swarm, and the radiation jets can be funneled through superconducting rings to extract the kinetic energy of the particles, something like a particle accelerator run in reverse.
Of course this will be an immense undertaking, a small Quasar might be the same size as our solar system, while large ones might have diameters measured in light weeks. In addition to the thermal and radiation energy, any structure built around a rotating black hole will be subject to "frame dragging", as the immense gravity of the black hole warps space-time around itself. The structures erected to capture the energy of the jets will have to extend astronomical distances - some jets can extend as far as a million light years.
PKS 1127-145, a highly luminous source of X-rays and visible light about 10 billion light-years from Earth. An enormous X-ray jet extends at least a million light-years from the quasar.
The other thing to note is that peak quasar activity in the known universe was about 10 billion years ago. In the current era, most galactic centres have been "hollowed out" by the action of the supermassive black hole at the centre of the galaxy - either absorbing all the easily available matter or "blowing" a bubble around itself through the radiation and energy when it was active. A project to capture the energy of a quasar would need to find a nearby active quasar, or make some sort of arrangement to feed solar system sized masses of gas and other matter into an inactive black hole. obviously arrangments would have to be made to allow the infalling matter to pass through the structures erected to capture the energy.
Another pitfall is getting to the central black hole. In the Milkey Way galaxy, the central black hole is about 30,000 light years from our location. It would take 30,000 years moving at the speed of light just to get to the construction site, and assuming you beam the energy back to Earth via a laser, it will take 30,000 years for the beam to reach the receiver. Other Quasar are millions to billions of light years away - it might even turn out that when you reach the site of the Quasar 10 billion years from now, the site has become inactive, much like today's Milkey Way.
So while it is possible in theory to capture the energy of a Quasar, there are going to be a multitude of practical issues in doing so.