先ずは憧れの作家の文章の呼吸をつかむためにひたすら筆写、丸写しをする。

、。

First, to grasp the rhythm of your favorite author's writing, you just copy it by hand, transcribing it entirely.

Literal

First [as-では] admired writer [genitive-の] writing [genitive-の] breath [object-を] grasp [purpose-ために] singlemindedly hand-copy, whole-copy [object-を] do.

呼吸をつかむ ('to grasp the rhythm/essence,' literally 'to grasp the breath') is a highly productive idiom for catching the 'flow' of something — writing, a martial art, a dance. ひたすら ('single-mindedly, solely, just') is an intensifier. 筆写 ('copying by hand with a brush/pen') and 丸写し ('complete copying') are formal/technical words for transcription. Reads like writing advice.