How to say to write freely/to express oneself as in Chinese 信笔

虽是信笔，但也曾下过工夫。 ( Although it is a letter pen, but also worked hard. )

信笔涂抹，稚拙可爱。 ( The letterhead is smeared, childish and cute. )

信笔疾书，文不加点，一挥而就。 ( Letter pen disease book, without adding text, just a swipe. )

信笔拈来皆文章 ( Stationery Articles )