Quick and easy way - pour a little very hot water in a mug, add a few frozen peas and let them sit for 5 mins. They will soften up and you can then squidge the pea between your fingers to get the skin off and discard that. Then chop the actual pea up into tiny little bits and offer just one weeny piece at a time.
I had to do this very thing this morning with my male betta who had gone off his food last night and this morning - ignored his favourite bloodworms and daphnia - so I tried the pea and he gobbled it up eagerly - gave him half a pea in total.
They go like that sometimes when they've had too much of the frozen, protein foods but the pea soon sorts their digestive system out and perks them up.
They also go off their food when they are a bit stressed - either from bad water quality or from being moved from one tank to another.
Athena