Master your writing skills in your mother tongue. Most literary translators, with very few exceptions, translate exclusively into their mother tongue, the language within best they express themselves and are most at home in. To translate a book, you have to write a book; to translate a play, you need to know how to write a play. The only way you can do this is write as much as possible in your language and continuously hone that skill. Most published translators are also published writers.
A rare exception to the rule was Israel's Leah Goldberg (1911 - 1970). She was European raised and educated, yet translated exclusively into Modern Hebrew, her third language, from six other languages. She however adopted Hebrew as her home and vehicle of expression and was a prolific writer exclusively in Hebrew as well.