We are in the process of doing this as we plan to retire early next year (March for DW/April for me) and will be moving fulltime from a house of ~880 sg ft 1st floor w/ unfinished basement of the same size into a Ram Promaster where the back cargo space is so approx 11.5 ft long, 6 ft wide, 6.25 ft high.
So, here is our process: Pick a room(bedroom, kitchen, library, ect).
Pull EVERY item out of that room and put in one spot. Physically touch every item and make the determination if you want to keep it or not. Start piles for goodwill.
So for example, we went through every item of clothing we had. The clothing and bedding that was worn/ratty/stained and we had just never got around to tossing(like socks that no longer had a matching pair) we tossed. The items we had bought and could no longer wear because we both have made a priority about eating healthier and gotten into better shape we either put in a bag for goodwill or gave to coworkers. Overall, we got 4 closets worth of clothing condensed down to comfortably fitting 2 with room to spare. once we retire, we will be able to get ride of a lot of work cloth and cut down on winter clothing as well.
We had 5 -ish book shelves stuffed with books(I say -ish because one in the basement had double rows in some places, and books stacked on top of the books). We pulled all these out, looked at every one and gave boxes to good will. For some of our authors we figured out what we could get on Kindle so we could give up the physical copies. We gave a box of books from one author that my mother likes to her as a Christmas present.
We went through the kitchen and pulled out all our containers, bowls, appliances, ect and tossed ones or gave away a bunch that we no longer use or need.
Same thing for all our boxed goods, we went through everything. We have cleaned up our diets a lot so we no longer eat certain things, so those got tossed.
Overall, I think we probably loaded up the car six or seven times with items for goodwill, and probably gave away 4 or 5 boxes of clothing, appliances, books and miscellaneous electronic cords and parts to friends, family and coworkers.
You will be surprised by how much you have accumulated. Even with us, we aren't very materialistic but after owning a house for 8 years and not having to pack up to move, you accumulate a lot of random stuff.
It's been great though, we have a lot more room and it's great to think that someone else is now using things that would otherwise go to waste.