When creating a brand new glossary for a specific client order or as a personal decision, translators should make sure to use the right reference material. Glossary creation should not be limited to simply listing source terms and trying to guess what could be the best translation in the target language. On the contrary, it is essential to know the context for each term, to fully understand the meaning without any ambiguity and to search for the exact corresponding translation.
In some cases, the client will already provide the list of English terms (with or without context) to the translators. In others, the request will simply be "to create a glossary". In both cases, the material that can be used will be, among others, various kinds of dictionaries; explanatory glossaries for finding definition of the source and/or target terms; bilingual material such as translated texts about the specific subject; documents that can help understanding and translating some terms; and so on. Remaining cautious about the validity of the reference material is of course essential as part of it might be outdated or not valid at all.
As always, the best option will be to make sure the client will revise and approve the final glossary.