According to this page , there are multiple Hentaigana express あ. How do I decide which to use?
As far as I am aware no-one has attempted to study usage patterns of hentaigana in classical texts, and from what I have been told by my professor it appears to be mainly an aesthetic choice on the part of the author. I think you can freely use whichever one you think looks the best in any given construction, without worrying about whether it is "correct" in that situation.
I hope I can recognize correctly what you asked.
Hentaigana are not used normally in contemporary Japanese. Instead, they use standard Kana (Hiragana and Katakana) and Kanji. Calligraphers often use Hentaigana in their production which is just for art and not intended for practical use.
Long long ago, men in Japan learned Kanji from China and there had not yet been Kana at that time. Kanji was so difficult for them that they made Kana from Kanji as phonogram which has no meanings but sounds. a Kana character is matched at a specific Kanji character (1:1). Thus Kana is specified by a character, but there are many Kanji of a same pronunciation therefore someone began to venture to use other characters for art (e.g. 'The Anthology of Myriad Leaves') who was women mainly. So Hentaigana has born.
Modern people don't use Hentaigana, but artists or trickers.