At first you need to collect all relevant data, this is:
1.-Impedance ratio, sec to pri or vice versa.
2.- Minimum frequency to be transformer with no too important loose/distortion.
3.- The maximum size allowable.-
4.- Power amount to be transferred.-
5.- The iron quality available.-
6.- Number of windings (in case you have more than two) for, feedback, for example.
There are several approach to the final device, but imho a great work is done in the Langford-Smith's Radiotron available in the web in pdf, and a work from an Argentine professor called Ladislao Koros from whom there is no work available in the net.