One way to stop air from leaking around a door is to use draft snake. The idea of the draft snake is that it acts as a seal between the bottom corner of your door and the threshold or flooring. This method may also require you to strike the door jambs with a mallet in order for an effective seal against outside air infiltration.
How to Patch Hole Where Door Threshold Meets Flooring
If you desire to patch up any hole where the threshold meets the flooring, you can accommodate that with weather stripping. You can purchase this at your local hardware store or home depot. Once you have the draft snake, place an end of it on one side of the threshold and pull it taut to the other side of the door. Cut off any excess using a knife or scissors so that there isn't anything hanging out over the edge of the threshold. Then cut a slit in the weather stripping at either end of where you want it to go on the threshold, making sure not to damage anything beneath it. Slide it down into place and attach each end onto each side of where you just put draft snake by screwing them into place through the opening of your door frame or jambs. This works great for stopping air from leaking in the winter months.
How to Stop Air from Leaking Around Door Knob or Handle
You can stop air from leaking around door knob or handle by using draft snake on each side of where they go. Attach one end of the draft snake on one side and slide it into place, then attach end onto other side. Then cut slit through weather stripping at either ends of where you want it to go on threshold above handles and attach each end onto each side of where you just put draft snake by screwing them into place through hole in your door frame or jambs. This works great for stopping air exchange when you are trying to minimize your heating bill during the winter months