To Create a Fusion Reactor you will need to firstly find a Gravitational Anomaly. Then put a Fusion Reactor Controller and follow the guide blocks. Put 3 Fusion Reactor Coil on each straight side of the circle. Decomposer can be directly put on each side of the Fusion Reactor Controller. Then fill every other block with . Once placed the reactor will show a circle filled with guides, that you can follow to complete the reactor. The reactor can be placed in either a vertical or horizontal orientation. This is done by rotating the Fusion Reactor block by either placing it from below or using a Tritanium Wrench. The reactor can also share its blocks with other reactors, this means that you can build a sphere consisting of vertical and horizontal reactors.
Once the Fusion Raector is complete, provide it with Matter to start producing energy. Hooking up 2 replicators on the left and right of the Fusion Reactor Controller and jump starting them with a battery, will enable the Fusion reactor to begin producing energy and charge the decomposers themselves. After that you will only need to feed in Plasma Matter.