Study the French alphabet, memorizing their chronological order and how the letters with diacritical marks are different from the regular ones. Practice writing these letters.
Memorize a certain number of basic French words every day, making sure to also memorize the gender noun to which it belongs. Study the contextual nuances of each word.
Study French grammar, such as prepositions, pronoun formality, verb formality and the fact that conjugations of such things as verbs and adjectives not only depend on number agreement, but gender agreement as well. The depth to which you study the grammar depends on what you want to write.
Memorize common French phrases, slang phrases and interjections such as "avec plaisir" or "with pleasure," "quel dommage" or "what a pity," "bonne chance" or "good luck," 'tais-tois" or "shut up," "allons-y" or "let's go," "pouah" for "ew/ugh," "silence" or "quiet," "regardez" or "look!" and "attendez" or "wait!"
Focus your vocabulary on the subject you want to write about in French the most, such as astrophysics, medicine, art, geology, ethics, ancient cultures, inventions, metaphysics or paranormal happenings. For writing rhyming songs and poetry, focus on words that rhyme. For writing in French on the Internet, study French Internet writing such as e-slang and emoticons (pictures made with keyboard symbols).
Write the French name of items in your living space on sticky notes, with one name on each note, and place it on the respective item.
Throughout the day, when you see things for which you know the French words, write them down. Practice thinking of the French words for things, until you do not have to think of the English words at all.
Write sentences in French, beginning with simpler sentences and progressing to more complex ones. Consider how a sentence will sound depending on how it is written; such as the alteration of the underlying sense of a sentence when word order is shifted, or how general inflection of certain phrases may color a sentence.
Look up French quotes, maxims, poems and songs or French recipes you want to try and copy them down. Go to restaurants where the menu is in French.
Study various artistic styles of penmanship. Practice one or several of the styles you prefer, perhaps combining them into your own unique way.