Well, there's no getting around it, exterior windows and doors, siding and trim do have to be sealed. But you know, there are products on the market today that will last up to 50 years. So if you do it the right way and use the right material, you don't have to do it very often.
If you choose a silicone sealant, select one that's paintable. Considering a new caulking gun? Make sure it's a dripless model. Window and door frames are most often in need of sealing. Eventually, old caulk loses its elasticity and cracks when wood expands and contracts. A scraper in this shape is ideally suited for removing the deteriorating sealant. Cut the tapered caulking tube nozzle so the opening is just slightly larger than the crack. Then puncture the inner seal at the base of the nozzle, drop the cartridge into the gun and you're ready to go.
Slow and steady is the name of the game here. Hold the gun at about a 45 degree angle and keep even pressure on the trigger. A bucket of water with a few drops of dishwashing detergent makes a good lubricant to smooth out the bead.
Or you can try this caulk smoother that leaves a finished look even the pros would envy. For wider cracks and gaps, this polyfoam caulk backing can be put in first. It will partially fill the gap, making the sealant go a lot further.
You can use a putty knife to push the rod into the crevice. Now for this kind of gap, you'll want to cut the nozzle further from the tip to produce a wider bead. Finish the bead with either caulk smoother or soapy water and your finger.
Now there's no mystery to caulking or sealing but there is well, a secret. Use high quality long-lasting materials. You'll be glad you did.