khadis- Hey @khadis-! Thanks for posting and welcome to the forum! I'd ask the client what do they need exactly and how do they plan to use the glossary.
I always create a glossary for all of my projects (for my personal use as it helps with consistency). If I was to bill my client for that service I'd bill an hourly rate because it can be a time-consuming process.
I normally create my glossaries as I go, but if it was a billable service I'd create a glossary before translating by simply going through the entire text, identifying repetitive terms and abbreviations and adding them to a simple spreadsheet.
Once I have a list of terms, I'd use a industry-specific dictionary (if available) and I'd maybe ask around on industry-specific forums to make sure my translations are 100% corrects.
Once the glossary is complete I'd convert it into a format that client wants. Depending on the size of your project and the complexity of your source documents glossary creation can take up to several hours.
I suppose it's also a good idea to ask the client to provide some sort of criteria to include terms in the glossary. For example, they might want to include technical terms that repeat at least 3 times in the text or include all abbreviations, etc.