Read your English text very thoroughly. Make sure you understand its aim, and decide your translation strategy according to the type of text to be translated. A technical piece of text must be translated with unfailing accuracy, whilst a piece of literature can be translated with higher regard to stylistic quality than source text fidelity.
Highlight any words or phrases you do not understand. Look these words up in a reliable dictionary, or on-line terminology database. If your translation is very specific, ensure that you are using resources that are adequate for your needs, for example, you will need to use a specific technical dictionary if you are translating an engineering text.
Re-read your text with your new phrases inserted. Make sure that they fit in contextually. Google terms, containing them in quotation marks to check their validity according to number of hits. A term with no hits may not exist at all. A term with lots of hits is more likely to be acceptable.
Create the first draft of your translation. Do not miss any details from the English text. Double check facts and figures. Leave your first draft to be edited later.
Thoroughly revise your draft. Pay particular attention to foreign language interference -- look for any constructions in your source text which seem to be awkwardly phrased -- as if they had maintained the source language sentence construction. Read your translation out to sound out any awkward phrases.
Spell-check the translation. Proofread the translation again, as some words escape spell-checkers. Ensure all grammar is correct. Ask a friend or colleague to proofread your final version.