One the main complaints of our patients is that the floss gets shredded or my gums start bleeding when I floss. I usually ask my patients to demonstrate their flossing technique for me. During the last fifteen years I have observed hundreds of my patients flossing, I have came across the same couple of the problems in the technique they are using.
First, instead of using a gentle, saw like movement to ease the floss in between teeth, most of my patients try to force, or shove the floss in between teeth. That usually causes trauma, and bleeding from the gum tissue. The solution is easy, use waxed floss, and just try to gently ease it through. For those who have a very tight contact between the teeth, you might want to consider using a dental tape, which is floss that has a tape like cross section, designed to navigate tight contacts.
Second, I noticed most of my patient after inserting the floss between the teeth, try to pull it up and out immediately, thinking that that spot has been cleaned. The proper way is to guide the floss between each tooth and under the gum line to remove particles of food, or plaque stuck between teeth. Ideally using a C-shape, the floss is curved around a tooth and placed under the gum line, and then gently moved away from the gum line, the floss scrapes the side of each tooth, and can also clean the front or back of the tooth. A clean section of floss can be used to clean each tooth to avoid transmitting plaque bacteria from one tooth to another.
Flossing should be done at least once a day. If you follow all the above for a month and your gum tissue is still oozing blood during flossing, this could indicate gingivitis, or periodontitis (gum tissue inflammation). A professional cleaning at the dentist might be needed.