Well you would take your current weight break it down so if you were 200lbs with 20% bf that means you are carrying 40lbs of fat. What I would personally do is take your bodyweight and bf% that you want to be and then eat for that goal.
The main basic component of fat loss(let's not call it weight because most don't want to lose weight they want to lose fat) is:
Calories out > Calories in.
So you must burn or use more calories than you take in. For yourself as a BB'r you want to preserve muscle and burn fat so Here's what I would recommend
1. Perform early morning HIIT cardio on an empty stomach upon rising - you can sip water your energy drink with BCAA's in it to help stop catabolism - Depending on how hardcore you want to be do this anywhere from 2 times a day (upon rising then after weights) - 5 days a week - 3 times a week.
2. Work out with heavy weights - Heavy as possible always trying to either add a rep or add 5 or 10lbs to the bar. Do this 3 - 5 times a week
3. Sleep well - at least 8 - 10hrs if you can
4. Macro ratio will be something like - P / C / F - 60 / 30 / 10 - try to stick to lean meats such as chicken / fish etc and consume 1g per pound of bodyweight a day.
5. Keep healthy oils up - Fish oils / flax oils / EFA's etc should be a daily supplement and keep your intake of these quite high.
6. FFS Don't Starve Yourself - Eat every 2 -3 hrs - people don't understand that the more you cut your food intake the harder it will be to lose that fat.
7. Water - At minimum 2Gallons / 4 litres a day. This will help you mobilize your fat as well. Reducing this intake or taking in alot of pop etc can seriously impede your progress.