As a Greek who lives in Greece: we generally don't translate our names in english and I haven't seen that being done with english names either. So, if your name is, for example, Sydney, theν you will write Σίντνεϊ. Although, for a lot of names we have a certain spelling form, like Ντέιβιντ (David), Κόρεϊ (Corey) etc. So, you don't just match sounds to letters, a lot of times they are certain. Also, it's better to take the current form because a lot of english names don't have a translation to begin with.

To perfectly answer to your question: A preliminary requirement is to either have a fundamental knowledge in speaking Greek or asking a native person who can help you to pronounce/ articulate the word properly. Otherwise the best alternative person is to refer to online applications translating words from one language to another such as "Google Translate". Hope this answer might have helped