...Supposing you are doing a Two Transistor Forward with Vin = 100V and Vout = 50V.
Say You want your duty cycle at max load to be around 0.4.
So that means the "effective vin" will be 50V/0.4 = 125V.
So you need your transformer to have a turns ratio of NP/NS = 100/125.
The best way to start to learn...when you really are starting learning of this ...is to go into the Ferroxcube transformer core catalog....pick any core at random...then shove literally 100 turns on pri, and 125 turns on sec....and see what it turns out like......measure up the turns with a wire table and see if the turns fit on your chosen bobbin for a start.
...Then calculate what your peak magnetising current will be........and if its too high, then choose a bigger core, or vary the turns , then re-calculate. Get a spreadsheet going and it will be quicker.
You can do it any way you like....
as long as...
You dont saturate the transformer
you dont cause core loss overheating.
You dont cause coil conduction loss overheating.
You get good enough coupling between primary and secondary
Your peak currents are not ridiculously high.
You are in CCM if thats what you want, and not DCM , if thats what you dont want.
Your proximity and skin effect losses are handled
Your turns fit the bobbin
You have enough pins on the bobbin to terminate your coils.
You have the right isolation spec.
Do you need triple insulated wire, or will you do margin winding...or just enamelled wire pri and sec.
..only by jumping in....doing it wrong, then edging it nearer and nearer correct, will this be learned.