。
She made elaborate preparations for the party.
Literal
She [topic-は] party [genitive-の] [for the sake of-ために] meticulous preparation [object-を] did.
入念な (にゅうねんな) is a na-adjective meaning 'careful, meticulous, thoroughgoing,' built from 入 ('enter') + 念 ('thought, attention') — literally 'thought put in.' Common collocations include 入念な準備 ('thorough preparation'), 入念な調査 ('careful investigation'), 入念にチェックする ('check meticulously'). It carries a slightly formal, written flavour compared to 丁寧な ('polite, careful') or 念入りな (the same idea with the morphemes reversed).