Welcome to the PF.
There are some pretty general design methodologies, but the actual design approach will depend on several things:
- Power Level
- Signal type (switching power supply, communication transformer, etc)
- Bandwidth
- Cost constraints (does it need to be cheap in high volume?)
- Safety issues (does it need to be designed as a safety barrier for AC Mains applications like switching power supplies?)
If you can give us an idea for each of these initial questions/items, that will help us a lot to guide you to some good transformer design tutorials. If you want to design a transformer for a 50kHz off-line switching power supply, that is very different from designing a wideband communication transformer for 50kHz twisted pair communication...
Also, what is your background in electronics and math? That also will help us to guide you the best. Thanks.