Mobile first has stopped being a slogan for me and become simply the way I start.
The idea is simple and it is uncomfortable. You design the smallest screen before the largest. Not after. Not as an afterthought squeezed in when the budget is nearly gone. First.
It is uncomfortable because it drags every hard decision to the front of the project. On a wide screen you can defer. There is always room for one more thing, so nobody has to say no, and a project can run for months without anyone deciding what actually matters. On a small screen you decide in the first hour, or you do not proceed at all.
Clients push back, and so do plenty of designers, and I understand it. They want to see the big beautiful version, because that is the one that feels like the real work. But the big version built first is often a promise you cannot keep on a phone.
So I try the harder thing. I ask what the one job is. I build for the thumb. Then I let the layout grow into whatever room it is given, and everything I add on the way up has to earn its place, because the thing already worked without it.
Mobile first is not really about mobile at all. It is about priorities, and about being forced to have them early. Which is, I think, why so many people resist it.