Remove the old caulking around the outside of the window frame with a putty knife and utility knife. Cut the caulking with the utility knife, and use the putty knife to scrape the caulking away. Clean the area with a dry rag, removing as much dirt and residual caulk as possible. Remove caulk from individual window panes if necessary.
Stuff insulation into large holes or cracks around the window. Alternatively, fill large cracks and holes with foam spray. Simply insert the nozzle into the crack and spray the foam until the foam is coming out of the crack. Allow the foam to dry for 30 minutes, and cut the foam flush with the wall with a utility knife.
Insert the tube of silicone caulk into the caulking gun. Cut the tip of the tube with a utility at the 1/8-inch mark. Most silicone caulks have graduated lines on the tip. Cut the tip at a 45-degree angle.
Start at a corner and run a 1/8-inch bead of caulk around the perimeter of the window frame where it meets the house, using the caulking gun. Make larger beads where necessary to cover larger cracks. Apply caulk to the inside corners of the exterior window frames as well. Place caulk around cracks and crevices where the frame connects at the corners. Dip your finger into water and smooth out the caulk. Run your finger down the caulk to give the caulk a uniform look along the window.
Apply caulk to the corners and edges of individual window panes as necessary. Run your finger along the caulking to smooth out the caulking into the edges where the window glass meets the window frame.
Tap in any loose nails in the window trim inside the house with a hammer. Run a bead of caulk around the window trim where it meets the interior wall.