Like any two different languages, you have to understand the MEANING of the first language, then translate that into the second language. Both languages may or may not have similar concepts -- most modern languages do, but we only have a relatively small amount of Dovahzul words and concepts given by in-game and approved resources (like the Prima strategy guide of the game), so you have to know some amount of Dovahzul vocabulary and sayings to be able to translate.

The steps you have to take are this:

1. What are the words in the original language sentence? You can use the Thuum.org word translator for this purpose.

2. For those words you can't immediately find the translations to, what words in Dovahzul can be used to say the same thing?

Keep in mind that not a lot of people have been regularly translating Dovahzul into English (the primary language of use - translating into other Western languages have similar steps). It's okay to ask questions (like in this Question Board). 

There aren't any in-depth resources for translating Dovahzul, besides this site and my Dovahzul blog (https://masterdovahzul.wordpress.com/ - check my blog posts from the beginning to get some tips -- unfortunately I don't have good organized learning materials YET)

TL;DR - Use translator when you can. For unknown words/concepts, just ask in the Question Board or try to rephrase and try again. Of course, accepting help is always better than going it alone...