I am primarily an emotional hoarder. Many of my possessions have worth (to me) in the sense that I am sentimental about them, they have memories attached, they link me to specific times in my life and it literally feels emotionally painful to get rid of them, like I'm getting rid of a part of myself. Additionally, I'm also one of those hoarders who can see a use for pretty much everything and hate things being wasted, so even when I don't have an emotional attachment to something, it can be hard to get rid of it if I can see potential in it. (It's not a great combination!)
I have had clear outs in the past, sometimes with help, sometimes not. It's extremely difficult, but here's a few tips for helping a hoarder have a clear out:
Take (digital) photos of things you are getting rid of. That helps when objects have memories attached. The hoarder knows they can look at the photos of the object and still have the attached memories or feelings. (In reality they probably won't ever look at the photos in the future, but it's comforting in the moment to have the option).
Do your best to donate anything that can be donated. It's much easier for me knowing that it could still be of use to someone than seeing my possessions chucked in the bin. Hoarders often hate the idea of waste.
Only focus on one category of thing at a time to avoid overwhelm (e.g, just craft supplies, just toys, just clothes, just kitchen implements). Don't try to do too much in one go.
Don't refer to possessions that they have emotional attachment to as "worthless", "crap" - to emotional hoarders, it's not about the price of the thing but the emotional weight attached to it.
This should be obvious, but just in case: the tendency to get really angry with the hoarder while you're clearing stuff out is not helpful. I've had this before (from two separate people in two unrelated clear outs) where I'd agree we needed to clear things out but as the person helping me went through my belongings they got more and more furious at me that I had so much stuff.
Don't guilt them. ("you shouldn't be like this", "what is wrong with you", "how can you live like this?" etc). It seems logical to a lot of people that trying to make the hoarder feel terrible about their hoarding will make them want to change, but it doesn't. In the same way that telling a depressed person to pull themselves together doesn't help. Hoarding is a mental health issue.
The best thing to do is acknowledge that it is hard for them, but that we do need to get rid of things and you are here to help them, not judge.