Set up a ladder or scaffolding as needed.
Remove old caulking from your siding. Use a putty knife to pry loose the caulk. Be careful not to damage the siding. The knife can easily gouge the surface of the vinyl. Once you have loosened it as much as possible, apply caulk remover directly to the remaining material. Make sure you are wearing gloves when working with the remover. Let it penetrate for several hours. Then use your putty knife to remove the softened caulk. Work it underneath and pull it loose. Dip a sponge into warm water and wash off any residue.
Remove any other loose debris, such as paint. Wipe the area with a clean, damp cloth.
Load your caulking gun and cut open the tip of the tube of caulk. The open tip should be the same approximate size as the vinyl seam you are working. Insert a nail or other sharp object down through the tip to puncture the seal of the caulk tube.
Start the caulk bead at the top of the vinyl seam. Hold the gun at a 45-degree angle. Slowly pull the gun down, applying steady pressure as you go. Be sure to fill the seam completely.
Use a caulk-smoothing tool or a spoon dipped in soapy water and smooth out the caulking. Time is of the essence. You need to smooth the caulk before it dries. This will give it a nice finish. Clean off any excess caulk with a damp cloth.