A repost from a friend on another forum, a very well respected, although natty bodybuilder:

this "guide", more of a stub really, is just to help clarify the simplest steps to creating, and maintaining, a deficited meal plan.

1- calc your bmr
calculate your basal metabolic rate (bmr) using online calculators:

-metric system: http://www.bmi-calculator.net/bmr-calculator/metric-bmr-calculator.php
-dumb system: http://www.bmi-calculator.net/bmr-calculator/

this will give you a magic number. yes, magic. your bmr number.
it's not your real bmr, only a doctor with a handful of tests (which can be costly) can give you a near accurate measurement, but this is enough.

2- add activity multipliers
this bmr is actually your rmr (resting metabolic rate), since it's not multiplied by any activity factors yet. you have to it, using any plain calculator.
most used multipliers + example of 2000kcal (kilo calorie) bmr:

2000x1.2 (sedentary/desk job)- 2400kcal << this is how much your body is burning off every non-strenuous day, if 2000kcal were your real bmr count.
2000x1.375 (strength/volume training, and no more than 45 minutes of 65%~75% HRM aerobics training)- 2750kcal << basically your workout day.
2000x1.55 (high volume strenuous training / over 1.5 hours of intense aerobics training / any hard physical activity job throughout the day or over 3 hours)- 3100kcal << a hard, strenuous, workout day's bmr count. don't be fooled by an hour of str or volume training sessions that were hard to be worth this multiplier, though.
2000x1.725 (extremely hard exercise, very strenuous and fatigue causing workout days, including extremely hard labor jobs)- 3450kcal << hardgainers start becoming more dominant in these days, and can easily make up for this multiplier even by doing str/volume training of just a moderate level, due to their high genetic metabolic rate)
2000x1.9 (marathon, contest, being chased by a friggin' grizzly bear)-3800kcal.

3- reduce by 20%
reduce what you got, after it's been multiplied, by 20% or a simple 500kcal. now you have a deficited, warped little magic number that will serve you in the meal plan to come.

4- break down to macros
the breakdown becomes more important the more you are into strength/volume/aerobics training. str/volume require more carbs overall, aerobic trainers (swimmers, marathonists and such) require more fats etc.
the breakdowns are formatted protein/carb/fat in percentiles. you can always break it down yourself, but those formats are generally known:

20/50/30
40/30/30
50/20/30
70/0/30 or 80/0/20 (keto'ers)

*note that im being neutral here, and even though i don't agree with several breakdowns (some listed above), i'll let you judge for yourself.
the reason i placed 30% fats on all breakdowns is because this actually is a very founded amount known to be optimal for gh/test production, which means it's optimal for everyone, bodybuilders powerlifters and even non-athletic normal people who just want to lose weight. i said i won't get into explanations so just take it as it is.

if you don't like to be given a breakdown just like that, do the following:
after being given your multiplied and reduced bmr, you must realize that each gram of protein or carb is amounted to 4kcal, and a gram of fat to 9kcal. knowing this, count your own body weight and multiply that weight by 1-1.5g, depending on how much protein you want to incorporate into your macros. 1.7g per kg of protein was tested to be of the highest potential in being synthesized by the kidneys for its amino acids' various purposes. bigger amounts yielded no better results whatsoever. after getting the amount of those grams, multiply by 4- that's the kcal amount of the grams. now for the fats, the process is reversed, take the whole bmr number and multiply by 0.3, that's 30%, and there's your fat amount in kcal. divide by 9 to get the gram amount. the rest is carbs, meaning whatever kcal you got from calculating the protein and the fats, reducing that and getting whatever's left in carb kcal amount, which is then divided by 4 to receive its gram amount.

5- choose your foods by integrating the macros you got
do whatever you want here, as long as it fits your macros (IIFYM). "healthy" or "poisonous", when it comes down to weightloss, it's about the amount, not so much as the quality.

i suggest http://www.fitday.com/ for all your logging & what's-in-my-food-macro-finding needs

6- weigh yourself every 2 weeks, adjust weight and repeat step 1&2&3
those don't take more than 5 minutes. after you get a lower number by losing some weight, it's best to relog onto fitday and see what every food you have in your meal plans consists of, so it's easy to remove (at least parts of it) in order to be adjusted to the new bmr. as a general thumbrule, every 2-3lbs is 100-150kcal less. take this suggestion with a warning that although it's common, it's also paradoxally almost never suited for you, so repeating the steps is probably the safest way. be sure to weigh yourself at adequate times, meaning that they match the conditions under which you were the very day you tested yourself priorly to this weighing session. if it was in the morning, do it in the morning, after a certain meal or meals, or before any meal- make sure to be as accurate as you can about those conditions as they may change the entire scale weight from good to bad, or from bad to illusionary

Q/A:
1) is doing aerobics gonna help me reduce weight faster? -it's gonna up your bmr activity factor, but you're still gonna have to eat at a 20% deficit. so not really.

2) can i have a cheat day or a cheat meal? if so, when? -1 cheat meal a week is fine. and don't overdo it.

3) i'm at an already fairly low bodyfat, yet even after i reduced my calorie intake 3 times now, the weight still won't budge, or even raised a bit. what do i do? -perhaps you need a refeed day, but i won't discuss it here. see all about it in leangains.com , and watch out for water fluctuations that may affect your weighing severly since your bf% is already low enough to be unseen by the weight scale when you do lose some.

4) i heard cutting carbs completely is the best way to lose fat, since carbs hold you down and stick to you like insta-fat. should i remove carbs? -i think that whoever preaches that should be staked for an over abundance of sheer stupidity. carbs are completely essential for several reasons: maintaining intracellular water muscle (given proper sodium-potassium and hefty water intakes), maintaining and raising strength levels, raising awareness and focus levels, removing kidney and liver load from overly eating proteins, and more. just because carbs got a low energetic expenditure, doesn't mean they have to have their name tarnished by superstitious nimrods.

5) should i get xx supps? -don't even get me started. do whatever you want here, though you're wasting your time & money imo.

6) i haven't been making gains in like forever been training 3-4 months in reality, should i intake like 8000kcal? -almost certainly not, don't be another victim to stupidity. it's a rarity to be a true hardgainer, and even those don't eat at such ungodly amounts. this is more of a bulking question, but i thought i'd add this here. simply follow the rules, only instead of reducing, adding 15% or 400kcal.