Things You'll Need
Smooth damp rag
Not only is caulking around exterior door jambs important for sealing out drafts; it will make your trim paint job look much more professional when you fill in all those unsightly gaps and seams.
How to Caulk Door Jambs
You need a caulking gun and some paintable caulking. Get a good brand of both. Cheap caulking guns won't cut off the flow of caulk from the nozzle so it will drip everywhere. Cheap caulking is hard to smooth nicely and might be lumpy or gritty. Cut the end of the nozzle just a little bit at a 45 degree angle, and either use the little wire poker on the caulking gun (if there is one) or a straightened out hook from a wire coat hanger to pierce the seal inside.
If you are painting unprimed wood, don't caulk until you have painted a coat of primer. First, caulking dries out faster when applied to unpainted wood because the wood absorbs moisture from the caulking. Second, once your door jamb is primed you can easily see what needs to be caulked. Every gap between the different pieces of wood, as well as where the trim meets the wall, should be caulked.
If the wood is already painted, it will be easy to see the gaps and cracks. Squeeze the handle of the caulking gun gently until you see the caulk start to come out. Now in one long smooth motion while holding the caulking gun at about a 45 degree angle, run a bead of caulking down the whole length of the trim gap. Wipe all the excess down with your damp rag, and follow this with your finger to blend it in. Use the rag to wipe your finger off after each swipe.
Don't over fill corners; just apply a little of the caulking material and smooth it into the corner and joints until there is no excess left. You will probably end up wiping as much excess away as you apply. That's OK. You want the seams to be completely smooth and blended in.
Wait at least a day or overnight before painting your trim. If you paint it before the caulking is completely dried, it will continue to shrink and crack your paint,
Cut the tip of your tube of caulking the smallest amount you think you might need. You can always trim it to make the hole larger but if you make it too large to begin with you can't make it smaller again. It's a waste of time sticking a nail into the nozzle when you are done. It will harden anyway. Just let the caulking material dry out at the tip and form a natural plug, and squeeze it out when you are ready to use it again.
Don't try to caulk the entire door jamb before going back and smoothing it out, do that as you go. Caulking starts to skin over quite quickly and then you can't smooth it out properly. Don't rely on caulking to fill nail holes. It shrinks too much and will show in your finished job. Use spackling compound instead.