If your daily fiber intake tends to be around 10 -12 grams, you don't want to go from 10 to 40 in one day. You wouldn't hurt yourself medically speaking if you do, but you could end up with serious social challenges...not to mention some unpleasant physical discomfort. So, let me suggest, on behalf of the author team, two different approaches to getting to your 40 grams of fiber a day. These are not etched in stone or research based, just reasonable suggestions.
Method One - By The Numbers
The first approach is for all the analytical, number types. Take the difference between where you are and where you want to be and divide that number by two. Increase your fiber intake by that amount for about one week. It could be shorter or longer than a week, depending on how your digestive tract responds to the increase.
So, for example, if you're at 10 and you want to be at 40, the difference is 30, divided by two is 15. So, you would increase your fiber intake by 15 grams a day for a week, adding whatever fiber-rich foods you want. Then increase by that same amount again, giving the body some time to adjust to the increased fiber
Method Two - Pace Yourself
The second approach is far more relaxed, for the free-spirited. It involves just increasing fiber-rich foods at your own pace, working your way up to 40 grams a day.
A couple caveats are in order at this point. Obviously, there is going to be different responses to different amounts and types of fiber-rich foods. But there are some foods that are notorious for creating more gas problems. The big one is beans. So, words to the wise: if you are not a seasoned bean eater, start slowly (1/2 cup a day) and work your way to greater quantities. There is research indicating that in a couple months the gas problems go away. Another problem for many people is raw cruciferous veggies, like cauliflower, broccoli, cabbage. If they don't work for you raw, eat them steamed. They'll still be nutritious and have the same amount of fiber.
Fiber Loves Water
Important reminder: fiber loves to attract water to itself and pull the water out of the body, so eating more fiber-rich foods means you need to drink more water throughout the day to replenish the body's supply.