Your biggest problem is getting the core material. Your best bet would be tape wound cores in the form of toroids.
In general you will start with an impedance ratio needed. Then take the square root to get the turns ratio. Then the number of turns needed is the amount that makes the inductive reactance several times the impedance at that side of the transformer at the lowest audio frequency.
Then you will need to size the wire to make the winding resistance a few % of the load value.
Then you will have to use insulation to withstand the voltages involved.