As a child, I remember I used to be really into making flower necklaces, so much so that I would forget the flow of time.
"As a child"＝「子供として」
"I remember I used to be really into"＝「私は～に没頭していた」
"making flower necklaces"＝「お花のネックレスを作るのに」
"so much so that I would forget the flow of time."＝「時間の流れを忘れるほど（没頭していた）」
★または、"so much so that I would forget how much time had passed." とも言えます。
アメリカ合衆国 
