Flat spins are really a feel move. Different planes need different inputs. Some you need to reverdse ailerons, some add lots of power, some draw out some elevator.... This is one of the hardest ones to tell someone how to do. THe best suggestion is to find a good pilot and get him to find the combination for your plane. For someone that knows the feel it doesn't take long to get. Once you know the combination it comes fairly easy.

One of the easiest entries for some is as a blender... And many times an inverted will flatten easier too. Enter it with an aileron roll with very slow speed on the downline. Maybe just one roll and then feed in the flatspin inputs deliberate but don't jerk them in. Then hold it and see if it developes, add some power. Sometimes it takes a revolution or two also when you are learning them.