Da iawn. Are they first language speakers?
I think there are lots of ways, but in general, I’d say it’s a case of getting a lot of exposure, in as many ways as possible. And by a lot, I think I mean much more than you would think (or more than I would have thought before entering into the language learning game).
While you are still doing a beginner level course (whether Course 1 or Level 1), I wouldn’t worry too much about actively learning vocabulary - you will have enough on your plate with the lessons. But no harm in getting at least casual exposure, e.g. S4C, Radio Cymru, and wherever you can find some spoken Welsh.
As you know, SSiW doesn’t really encourage reading or writing Welsh in the early stages. How long this voluntary self-denying ordinance lasts is left entirely to the individual learner. While I was still doing Course 1, I didn’t go out of my way to read any Welsh whatsoever (even the official course notes), and made a rule to myself never to write down anything from the course.
However, words I came across on the radio or on S4C, or in occasional meetups, etc, were fair game, and I put those into a notebook, and looked them up when convenient..
For Welsh at least, I didn’t use any particular word-memorising method (not strictly true, but won’t go into that here).
However, I did eventually start reading, and eventually found some books aimed at learners - the “Nofelau Nawr” series, and “harvested” loads of useful words and phrases from those.
Reading is usually reckoned to be a very good way of growing vocabulary, and I’d agree, but I’d also say, it shouldn’t be the only way. In line with the SSiW philosophy, one should probably be doing at least as much listening as reading, and purists would say as much speaking as listening, although that’s not alway quite so easy.
I’d say try not to make vocabulary-learning/growing a chore, and try to keep it to being a pleasure.
I used to encounter a word I didn’t know, and would have a sinking feeling - “oh no, another word to learn”. I gradually managed to re-frame this, and started thinking - “oh good, another word to add to my store-cupboard!”.