First of all, I don’t recommend making vocabulary your “primary” focus. This depends on your goal, but for most people, it won’t help that much.
Here’s an extract:
If your goal is to speak well in a conversation in general, you’re better off focusing on larger blocks of English or “chunks”, because these are what will help you sound fluent and natural when you speak.
I won’t bother talking too much about chunking here, but if you’re interested I’ve got a free training course that teaches about this (go here).
This said, if your goal is to get good at a very specific topic, then yes, you might need to focus on vocabulary.
For example, say you need to buy some bike parts or accessories in a bike shop. You’re not going to be able to talk about all this if you don’t know the specific language (e.g. “pannier”, “chainstay”, etc.).
So the point is, vocabulary learning is definitely better done in specific topics.
Now, as for the how to:
Whether you’re learning chunks for conversation or specific vocabulary, the best way is to learn from context. Find high-quality samples of English (whether good materials that someone has designed for English learners or “real” English) and learn what you see (this is often called “data-driven learning” – i.e. learning from real data and mimicking that, rather than learning from lists and trying to create sentences yourself). Avoid memorising from lists, as this is pretty much useless for actually remembering what you learned.
As well as chunking, there’s a section in the free training I mentioned about what the best materials to use are (and what to avoid).
Hope that helps.
Dr Julian Northbrook