Start with little sessions and build on that. Even if it’s something small like breathing meditation, reading, or working on a small project. Spend just a minute (or two or ten), whatever you can manage. Then add a minute or two over time until you’re up to bigger blocks. Simple but it works for me.
This might sound weird but when I’m heating something in the microwave or sitting at a stoplight (stuff lasting a minute or two), I’ll see how long I can manage to focus on the seconds that pass without my mind wandering. If I slip, no worries but see it as a chance to practice holding attention to the moment.