How do you stop fantasising about real girls like your ex or imaginary girls..

Keep yourself busy.
Learn something new, work out and focus on becoming a better human being.
It helps you should try

 

Good.
Keep up the good habits, eventually your fantasies will start to fade away.
Don't ever quit

 

As per my experience, I fantasized a lot in first one week and then gradually it reduces in the upcoming days.
I believe that keeping yourself busy and a strong mindset to make yourself a better person works a lot.
Try adding cold showers to your routines, it works well.