CT's Carb Cycling Codex article

First Step: Calculating Your Basal Metabolic Rate
Your basal metabolic rate (BMR) simply means the amount of energy used by your body during a 24-hour period if no activity is performed. In other words, if you're inactive for 24-hours straight, you'd still "burn" the amount of calories equivalent to your BMR.
Your BMR is a function of your size, sex, and age. It's also influenced by your metabolic status (hypo or hyperthyroid state for example). We can calculate BMR with the following formulas (by Harris-Benedict):
For Men
BMR = 66 + (13.7 x weight in kg) + (5 x height in cm) - (6.8 x age)
So for a 30 year old bodybuilder of 220lbs (100kg) at 5'11" (178cm) it comes up to:
BMR = 66 + (13.7 x 100kg) + (5 x 178cm) – (6.8 x 30)
BMR = 2122 calories per day
For Women
BMR = 655 + (9.6 x weight in kg) + (1.7 x height in cm) - (4.7 x age)
So for a 28 year old figure girl of 132lbs (60kg) at 5'6" (165cm) it comes up to:
BMR = 655 + (9.6 x 60kg) + (1.7 x 165cm) – (4.7 x 28)
BMR = 1380 calories per day

Second Step: Factoring in activity level
The amount of calories found using the Harris-Benedict formula is what your body burns every day, even if you do nothing all day. Obviously, the more active you are the more you'll burn fuel. So, energy expenditure will be increased when your activity level goes up.
To get an adequate estimation you need to multiply your BMR by an activity level factor:
Activity level factor
Activity level
1.0
Sedentary
1.2
Very light activity
1.4
Light activity
1.6
Moderate activity
1.8
High activity
2.0
Extreme activity

By sedentary we mean doing nothing all day (sleeping and watching TV).
By very light activity we mean doing nothing physical. Working a desk job or on a computer and not performing any type of physical activity during your day.
By light activity we mean having a non-physical job (desk, computer, etc.) but performing some sort of physical activity during the day (e.g. above average walking) but no hard training.
By moderate activity we mean having a non-physical job, performing some sort of physical activity during the day, and including a daily workout session in your routine. This is where most of you are at.
By high activity we mean either training plus a physical job or non-physical job and twice-a-day training sessions.
By extreme activity we mean a very physical job and daily hard training.
So if our 220 pound bodybuilder with a BMR of 2122 calories/day is moderately active, his daily energy expenditure is bumped up to 2122 x 1.6 = 3395 calories per day. This is the amount of food to consume to maintain present body weight.

Third Step: Adjusting caloric intake to your goal
To gain muscle you should ingest more calories than you use up each day. To lose body fat you must do the opposite. A 20% increase or decrease seems to be ideal for most individuals. This isn't a drastic increase/decrease, so it shouldn't lead to excessive muscle loss or unwanted fat gain.
Our sample bodybuilder has a daily caloric expenditure of 3395kcals/day. If he wants to gain muscle mass he should bump his caloric intake up to 4074kcals/day. And if he wanted to lose fat he should decrease it to around 2716kcals/day on average.
Note that depending on your body type and metabolism, you may need to adjust these figures. Ectomorphs will need to increase caloric intake more than 20% to gain muscle maximally (around 30% is best for them) and they should decrease it less when trying to lose fat (by 10% instead of 20%). Endomorphs should only increase by 10% when trying to gain size, but lowering it by 20% is adequate for them when trying to lose fat.
For example, if our 220 pound bodybuilder is an endomorph he should ingest 3734kcals/day when trying to gain mass (instead of 4074kcals/day).

Fourth Step: Setting nutrient intake for the "moderate days"
Protein intake should remain stable during all three types of day. At least one gram per pound of bodyweight is necessary, but I recommend 1.5g/pound of bodyweight for better results (so 330g/day for a 220 pound individual).
When trying to gain mass, the carbohydrate level on the "moderate" days should be the equivalent to the protein intake. So in the case of our 220 pound bodybuilder, that comes up to 330 grams.
So he's now consuming 2640kcals/day (1320 from proteins and 1320 from carbs). Let's say that he's an endomorph. If he wants to gain muscle mass, his caloric intake should be around 3734kcals/day. So he has 1094kcals to consume in the form of fat, preferably good fats. Since fat has 9kcals/g, this comes up to 121g of fat per day.
So to recap, our endomorph bodybuilder of 220 pounds wanting to gain size should consume the following on "moderate" days:
330g of protein
330g of carbs
121g of fat
If he desires to lose fat, carb intake on the average days should be set at 1.25g per pound of bodyweight. For our sample guy that comes up to 275g per day.
Protein intake is kept at 1.5g/pound of bodyweight (330g in our example) and the rest of the calories are made up with fat.
In the case of our endomorphic bodybuilder who should consume 2716kcals/day to get ripped, we come up with 2420kcals from proteins and carbs, so he has around 300kcals to consume in the form of fat, or 33g/day.
To recap, our endomorph bodybuilder of 220 pounds wanting to get ripped should consume the following on "moderate" days:
330g of protein
275g of carbs
33g of fat

Fifth Step: Setting nutrient intake for the other days
Protein and fat intake remains constant during the week. Only carbs fluctuate up and down. During high(er) carb days, bump carbohydrates to 125% of moderate days. During low(er) carb days, intake is lowered to 75% of the moderate days.
To continue on with our example, our 220 pound bodybuilder would consume:
1) When trying to gain mass:
High(er) carb days = 330g protein, 412g carbs, 121g fat
Moderate days = 330g protein, 330g carbs, 121g fat
Low(er) carb days = 33g protein, 247g carbs, 121g fat
2) When trying to lose fat:
High(er) carb days = 330g protein, 344g carbs, 33g fat
Moderate days = 330g protein, 275g carbs, 33g fat
Low(er) carb days = 330g protein, 206g carbs, 33g fat

Sixth Step: Adjusting intake as the diet goes along

Warning: In my honest opinion, no one who's trying to get muscular should follow a restrictive fat loss diet for more than 16 weeks in a row. And most people would be better off using 8-12 weeks of dieting.
More than that and you're bound to lose muscle mass or at least limit your capacity to gain muscle mass. If you haven't gotten to the degree of leanness you wanted after 12 weeks of dieting, take 4 weeks "off" of your diet (continue to eat a good clean diet, but increase your calories) and then go for another dieting period.
When trying to lose fat, you'll need to eventually lower your calories as your body gets used to your level of food intake. With carb cycling this is less of a problem since carbohydrates and calories fluctuate. But still, every 3-4 weeks you'll need to decrease carbohydrates and calories slightly to continue losing fat at an optimal rate.
However, you shouldn't make any drastic cuts, as this is the reason most people lose muscle during a fat loss diet. I suggest dropping around 20g of carbs per day every 3 or 4 weeks. For example, if you're consuming 344g, 275g, and 206g, you'd reduce it to 324g, 255g, and 186g. If fat loss hasn't slowed down, there's no need to reduce anything though.
When trying to gain mass, there's no set pattern of increasing caloric intake. If after 2-3 weeks you haven't been gaining size, increase protein and carb intake by 25g each. At first, add this amount to your post-workout meal. If after another 2-3 weeks there's no change, add the same amount to your breakfast. If you're still not gaining, add some more to your post-workout shake, etc.

Here is a link to this article as well as more info regarding this:
https://www.t-nation.com/readTopic.do?id=811783