Get a plan, and then work in order. Your parent chooses what goes to the new home, family chooses what they want to keep, and the rest goes to an estate sale, consignment, donation, or clean-out. Start before the move-in date.
After decades in one house, this is usually the longest part of getting it ready to sell. Working in order keeps things from leaving the house before the right person has seen them.
Get the floor plan and room measurements from the community, along with any rules on furniture. Measure the pieces your parent wants to bring. Your parent decides what goes. Move those items first, so the new home is set up and familiar on day one.
Have each family member list what they want. Where two people want the same thing, take turns choosing. Photos, letters, and keepsakes can be scanned or divided. Items for family out of state can be packed and shipped.
Go through drawers, closets, and boxes before any vendor arrives. Look for:
Keep the papers together and give them to whoever handles your parent's finances. Shred only once you're sure nothing is still needed.
In my experience, plan on several weeks from the move to an empty house. The size of the house and the number of family members involved can stretch it. Estate sale companies are often booked weeks out, so call them before the move-in date.
I connect you with trusted local vendors for the estate sale, clean-out, and repairs. You make the decisions and hire them directly. I help you manage the process and keep everyone informed.
Once the house is clear, the next decision is what to fix. Should I renovate before selling my parent's house? covers that, and a Home Audit shows which repairs are worth doing.
For more on the whole process, see how I approach helping a parent move and sell.