This is ultimately a lifestyle change.
You have a serious but achievable journey ahead.
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Get rid of trigger foods. Toss or donate to neighbors.
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Make sure you don’t have any appointments or dates ahead where you’ll be expected to eat or be tempted.
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Stock up on water
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Have some pink salt and/or electrolytes handy
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Have some bone broth ready for refeed and read up on refeed. For a 7 day fast you’ll want a 3 day refeed.
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Put stickies up. There’s a video on youtube about that
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Define your “why” and write it on paper. Stick that on your fridge and the bathroom mirror and all over.
There’s more but that will get you started. 
You may lose as much as 15 lbs - but keep in mind some of that will come back on the refeed. It’s water weight.
What you really lose will be 1/2 a pound of fat a day - possibly 2 as it sounds like you are over 200.
If you’re not going in fat adapted (low carb) you may get a headache. Just buckle down as it will go away.
Welcome here and good luck!
PS: we have a winter solstice slim down challenge if you’d like to join. You make your rules - we are all fasting our own schedules through December 22. 