How can I work from/save videos to an external hard drive?

How exactly do I do that?

I want to make short YouTube videos, which I record on my phone. One is often 10 minutes long, and after editing in Final Cut and saving, it's around 800MB (the file size is that large). I make videos every day, and my laptop is already full in terms of storage.

I also have external hard drives and have been using them as backup drives.

How can I work directly from there now? So that my videos from my phone are transferred/saved there and saved to FinalCut, rather than on my laptop?