1. Read all that you can about the differences between the U.S. legal system and the civil law tradition. Two useful introductions:
John Henry Merryman and Rogelio Perez-Perdomo, The Civil Law Tradition: An Introduction to the Legal Systems of Western Europe and Latin America. 3rd edition. Stanford Univ. Press, 2007.
Mary Ann Glendon et al., Comparative Legal Traditions. 3rd edition. West Publishing Co., 2008.
2. Realize that not everything you need is on the Internet; reference books are still vitally important.
3. Beware of British terminology in the bilingual dictionaries:
High Court (a court of first instance in England, but used by American journalists to refer to the United States Supreme Court)
locus standi (this is called "standing" in the United States)
Rules of the Supreme Court (this is the equivalent of the Federal Rules of Civil Procedure in the United States)
4. Sometimes terms of art are best left untranslated (such as the "Conseil d'Etat" in France).
5. Don’t assume that legal Latin should be left untranslated (for example, "res judicata" is called "cosa juzgada" in Spanish and "chose jugée" in French).
6. Trade legal documents with a translator who translates in the other direction so that you can see what legal language sounds like in your target language.
7. Arrange documents by document-type (contracts, court judgments, etc.) and collect boilerplate.
8. Just because you’re translating a legal text does not mean that you should ignore the rules of stylistics in the target language.
9. Draw diagrams and flowcharts to help you understand the situation reflected in the document you’re translating.
10. Think! Use your common sense.
If you read the fuzzy word "xu" as "ou" you will get "The buyer must pay by February 1st or later" (which is a highly unlikely rule) but if you read it as "au" you get "The buyer must pay by February 1st at the latest" (which makes perfect sense).