Absolutely need flaps. I would not reuse old flaps unless they were in great condition. If you are replacing old tires and tubes I usually let the new tires sit in the sun for a few hours to soften them up. Once the new tube is in the tire cavity I give it a shot of air, not a lot, just enough to unfold the tube to fit the tire. And starting with the valve stem work your way down each side with tire spoons (don't use screwdrivers, or anything with sharp edges). Getting the tube into the tire cavity is a must. If it gets pinched between the rim and tire it can be trouble also. A little soap around the rim is helpful also. Be patient and good luck.