1. Ideas/Information: Jot down in French relevant words you know, and ideas and information you want to include. Try to think in French as you do this, instead of translating. Group the words and phrases that go together, and think of conjunctions that might link them (et, mais, tandis que, parce que....)
2. Vocabulary: make a list of English vocabulary words you still need to know after Step 1. Look up and write down the French equivalents, including genders and plurals of nouns. Use your dictionary carefully: select the best one from the list of alternatives given; if in doubt cross-check with the French-English section to make sure the word says what you want. Note any usage examples given in your dictionary. Beware of translating idioms literally!
3. Write a first draft on a computer (for easy editing) using short, basic sentences. Click here for information on how to type the French special characters on a computer.
4. Expand this first draft:
(a) add adjectives (including comparatives and superlatives), adverbs, descriptive phrases, prepositional phrases (with what, after what, for what, since when,...). Vary your verbs; use modals to say what can, should, must happen; use the subjunctive where appropriate.
(b) Connect sentences with conjunctions; convert some simple sentences to relative clauses(advanced). Remember that subordinate clauses and relative clauses must be set off by commas.
(c) Vary your sentence structure--instead of always beginning the sentence with the subject, put another sentence element in the first position. Time expressions, adverbial conjunctions, prepositional phrases , expressions of opinion (je pense que...) and expressions of probability (probablement, peut-être que...) work well in first position. If you want to stress a subordinate clause, use it to begin the sentence. You should also vary the length of your sentences.
Note:
(1) Unlike in English, no comma is used in French when something other than the subject is in the first position.
(2) if you construct your longer sentences from short ones in this way, you can also avoid the danger of producing confusing sentences.
(d) Occasionally, you may want to use direct or indirect rhetorical questions (e.g. "Pourquoi est-ce si important?")
5. Have someone (your correspondence or tandem partner) proofread your expanded draft.
Source: http://www.lsa.umich.edu/german/hmr/schreiben/schreibstrategien.html