You will probably have to dig up the old wood and mush that is there now, at least the biggest stuff.
A garden fork works well to pry up chunks submerged like that.
Maybe smash and bury the rest semi-level with a sledge.
Trying to lay patio blocks on that uneven junk pile is not going to come out well.
Then you can bring in bags of paver base and level it with a metal rake, and compact that with a hand tamper, or just walk around stamping your feet to compact it, smooth it again, lay down blocks.
Something I mentioned to another member is to inquire at local churches or charities about any groups that might be able to donate labor, to help fix up a house for someone in need.