I’m a huge proponent of Dana K White’s approach to declutterring and I’ve used it with my dc and dm.
She has a 5 step, no mess method and it’s brilliant. It starts with looking for trash (their definition, not yours). Then you look for things that belong somewhere else and take them there right away (no piles). Next look for obvious donations (again their definition, not yours). And then you’re down to the sticky things so you ask where you’d look for it first and take it there right away. And if the answer isn’t obvious ask if you’d remember you had it (eg would you actually even look for it or buy a new one).
If you start with a small area like a shelf or drawer, it’s a great way to build confidence and trust that you’re not going to ride roughshod over them. I love doing this because it prompts a bit of storytelling and reminiscing. It’s slow to start but if you can sit tight and not jump in and take over, it gains momentum. You don’t really have to go in order so if they can’t recognise that stuff is rubbish at first, or should be donated, don’t sweat it. The trick is to keep going and not get bogged down. If it’s hard, then skip it and work on something else.
Pick something that matters to them - an area that causes them difficulty right now. Once you build declutterring momentum it will snowball, but work on what matters to them now, not an abstract future issue like moving if there isn’t an actual plan.
With my dm, I do the legwork, put stuff where it belongs and take away rubbish and donations. To begin with she absolutely couldn’t recognise that things like her collection of 150+ coloured glass bottles that was scattered across the house was bloody useless a bit excessive but I didn’t push it. We had agreed they belonged on a shelf on a display cabinet and after a while she weeded them a bit, then a bit more, and then reduced the collection to five that have particular significance. It would be so easy to fall out over stuff like this, but I think if I’d pushed her that 150 might have reduced to 130 and she wouldn’t let me into other things for fear of what I’d throw away on her.