How do you translate a sentence?

The first thing to do when translating a sentence is to read it over at least once, to get a general gist of how long it is, how many clauses there are, in short, how easy or difficult said sentence will be. Let us use, as our example, a standard Classical Latin prose text - that of Caesar's Bellum Gallicum.
Gallia est omnis divisa in partes tres, quarum unam incolunt Belgae, aliam Aquitani, tertiam qui ipsorum lingua Celtae, nostra Galli appellantur.
Now, the next important step is not to freak out. Many texts might have such tribes or peoples as Belgae, Aquitani, Celtae, whose names might not be familiar or recognisable. It is important not to stress about this, or any other words unknown, as the most key aspect is to work out how the sentence works, and the rest will fall into place.
First find the subject of the main clause, in the nominative. Then its number and then gender. It is clear, then, that Gallia is the subject, feminine singular.
Now we find the verb. Here it is est...divisa, agreeing in case, gender and number with Gallia, in the passive. Gaul is divided.
Now we look for the object. With a passive verb, there is none, so we look for a prepositional phrase that functions as added information. Here, it is in tres partes. Gaul is divided into three parts.
Now we've dealt with the main clause, we see that quarum indicates a subordinate clause. First, find out what the subordinate clause is referring to. It is in the feminine genitive plural, referring in the main clause to partes, which is feminine plural. Gaul is divided into three parts of which...
And so, with the subordinate clause, we repeat the process. Find the subject. Belgae. Find the verb that agrees with the subject. incolunt. Find the object. unam. one part of which the Belgians inhabit. And fill in the rest!