First you have to find the best berries,
Enigma, for Spicy
Micle, for Dry
Custap, for Sweet
Jaboca, for Bitter (all of these are from special events)
And for sour there isn't any as good, but,
Colbur or nomel berries are good choices.
Practice mixing at the house in hearthome.
Next you have to find friends who have the same berries. If they are good at mixing too then the poffin will turn out awesome.