Step #3: Edit + Sort
Now that you have a idea of how you want to live and an idea of the new space itself, it's time to go through your things. This is so you get reacquainted with what you have, edit out (donate, sell, give away, recycle, and/or trash) what's not adding to your life, and get things sorted so you can move onto the next step, mapping out the new space! Plus the less you have, the less you have to move.
EDITING: I believe our things are tools to help us live the lives we want to live. Ask yourself "does keeping this inhibit or enable me to live the life I want to live?". Don't get rid of something just for the sake of it, or because you haven't used it in X amount of time. Keep things you'll actually use (even if only once a year) and are worth the 'cost' of keeping; we pay by square foot here in NYC, so we literally pay to store things in our apts.
SORTING: Group like things together; all kitchen stuff, office supplies, clothes, tools, batteries, vases, etc. A key concept to being organized, this also helps you edit. Example: seeing a vase on it's own, it's hard to know whether to keep it, but seeing all your 15 vases together you can see maybe it's time to let go of a few. Use shopping bags or designate surfaces for each category and use labels to keep things straight (believe me, it helps!).
It's never too soon to do this as it can take quite some time depending on how much you're starting with.
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