Dad has been saying the front room is a little cold near the door for a while now, and he has been moving a rug to block the draft. The fix is usually a few strips of weatherstripping and an afternoon, if the door itself is sound. This is how to find the gaps, buy the right parts, install them in order, and test the result.
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What to do tonight
Do the draft check with him sitting in his chair, so it stays his house and his project. On a windy evening, turn off the lights in the room with the door, and have someone stand outside with a flashlight. Move the light slowly around the edges, top, and bottom. Any light showing through is a gap. Then, from inside, run the back of your hand along the sides and along the bottom to feel for moving air. Mark the worst spots with a strip of painter's tape.
Check the door before you buy anything
Weatherstrip will not fix a door that does not close square. If the door sticks, rubs at the top corner, leans, or needs a hard push to latch, fix that first or call a carpenter. Note the sill too. Look for the small drainage openings at the bottom of the frame, and leave them open. Sealing them traps water, which is a worse problem than a draft.
Match the strip to the gap
Each gap needs a different part. The sides and top usually take a compression or V-shaped strip on the frame. The bottom takes a door sweep on the inside face of the door, or a threshold seal on the floor. The strip that meets the door when it closes, on the latch side, may need a thin stop-side seal.
Measure the width of the gap with a ruler or a folded piece of cardboard. Read the product label for three things: whether it is rated for exterior use, the temperature the adhesive needs to set, and the thickness it compresses to. A strip that is too thick will stop the door from latching. Buy one size up only if the label says it compresses enough.
Install in this order
Pick a mild, dry day that meets the adhesive temperature on the label. Wipe the frame with a dry cloth, then clean it with rubbing alcohol if the label calls for it. Let it dry completely.
Peel off the old strip and scrape off the residue. Old residue stops new adhesive from bonding.
Measure each run: the top, each side, and the bottom where the sweep will go. Cut each piece a little long, then trim to fit.
Install the top first, then the sides, working from the top down. Press the strip on firmly. Do not stretch it, because it will pull loose.
Install the door sweep last. Close the door, then mark the sweep where it just touches the threshold. Fasten it to that line. A sweep set too low will drag and wear out quickly.
Close the door slowly. It should latch with normal hand pressure, without shoving, and the sweep should touch without dragging.
Repeat the flashlight test from tonight. The strip should compress evenly without crushing, and no light should show.
Check it again after the first windy day and after the first deep cold snap.
Mistakes that cause trouble
The most common ones are a strip that is too thick, a sweep set too low, and a drainage opening that has been covered. A door that will not latch after the job is almost always the first.
Who should do the work
If the frame is sound and the gaps are small, one person can do a few doors in an afternoon. If the door needs planing, the frame is soft or damaged, or the work involves a ladder at height, call someone. Dad should not be the one on the ladder, and you should not be alone on it either.
For a few drafty doors on a sound frame, this is a good job to finish before the first cold snap. Once the strip is on and the flashlight test comes back clean, the rug can go back where it belongs.