1. Cut off your left arm, and donate your liver.
Here's the thinking: Your liver is roughly 3.2 pounds (source), and one arm is about 5.335% of your total body weight (another source). For a 150-pound person, that means that an arm weighs about 8 pounds. SO, an arm and a liver together would add up to about 11 pounds!! YAAASS. Easy peasy!
2. Lob off your right arm, and remove your brain.
According to this illuminating and not-at-all sketchy article, your brain weighs 3 pounds. We already know an arm is about 8 pounds... And boom bam bowwie, 11 pounds lost in NO TIME.
3. Remove both feet, one hand, and both of your lungs.
More fun with math: foot (2 pounds) + foot (2 pounds) + hand (1 pound) + lungs (5 pounds, at 2.5 pounds each) = 10 POUNDS, POOF GONE.
4. Get rid of both kidneys (0.5 lbs), your small intestines (3.5), one lung (2.5), and your entire lower arm, from the elbow down (3.25). And then take a really big poo (about a pound, give or take).
5. Remove all the skin from about half of your body.
Your skin is about 16% of your body weight, according to this very reliable source*. If you weigh 150 pounds, that's 24 pounds!!!! About half of that is 12, which gets you past your goal.
*Not actually reliable
6. Remove the skin from the other half of your body.
See above math.
7. Off with one-half of one thigh!
8. Cleanly detach your head from your body.
PERFECTION. Also, source.
9. Or you know what? Just don't, actually. Don't aim for a goal like this at all. Be good to yourself, instead.
Aiming to lose X amount of weight in Y amount of time isn't a healthy way to approach diet and weight management — and it's also setting yourself up for failure.
If your goal is to lose weight for health-related reasons (say, you're dealing with diabetes, high blood pressure, joint and back pain, or other such issues): Work with your doctor and a registered dietitian or nutritionist to come up with an eating plan that meets your needs.
If your goal is more along the lines of wanting to feel better in general: Try to adopt some healthy living habits, rather than focusing on the number on the scale. Eat more yummy, healthy foods that give you fuel and make you feel good and energized. Get plenty of exercise, or at least make an effort to move and use your body each day, to whatever degree you can. You might get some motivation and ideas here. And get good sleep each night. Some pointers to help you sleep better here, if you need them.
Weight loss might happen as a result of making these lifestyle changes...but even if it doesn't, you'll still feel healthier, stronger, and more energized than you did when you started. And that's a million times better than starving yourself, obsessing over every last calorie, and/or chopping off body parts willy nilly, IMHO.
In case it wasn't clear: Items 1 — 8 are pure sarcasm. Do NOT chop off any of your body parts or remove your organs.