Step 1: Throw away all the things you don't use. Like, literally, just throw them. In the dumpster. Behind your house. In the alley. Where the raccoons live. Don't look at them, just go.
Substep 1: Sort things into neat little piles. Like, really neat. Like, you-just-graduated-from-college-with-a-degree-in-Interior-Design neat. Don't get distracted by the dust bunnies.
Subsubstep 1: Label each pile with a sharpie. In a color that contrasts with the background. So you can see it, duh. Don't label the dust bunnies, though. They'll just get all upset.
Step 3: The Part Where You Put Everything Back Where It Belongs Advanced Decluttering Techniques: Because You Clearly Need More Help Decluttering 101: For Kids (Because, Why Not?)