The Fusion Chamber, a multi-block structure made of tungsten plating and fusion wall, is shown how to construct by getting the fusion plans from a villager for the price of 1 diamond and an emerald. These plans are then put into the projector - once in the projector glowing blocks will appear in front of it. That is where you place the Fusion Walls and Tungsten Plating.
Once fully constructed the Fusion chamber requires 2 elements of your choice and electricity with which to power it. Once fully made and powered, the chamber will combine the 2 elements into a new element. It adds the atomic number of both elements, consumes them and produces one element with summed atomic number.
Good luck, have fun - and remember the radioactive elements decay, and can kill you.
And remember: with great power, comes great responsibility.