Finding a way to reach into deep corners when you're caulking can be quite a trick, but here's a quick tip for you:
The nozzle on a tube of caulk may be too short to reach into deep corners and narrow spaces, but you can extend its reach with an ordinary plastic drinking straw. Just slip the straw over the nozzle tip and secure it with a little duct tape.
Then cut the straw long enough to do the job, and you're ready to go to work in the corner or crevice that needs to be caulked. You may need to guide the straw with one hand - if that's possible - as you squeeze the caulk gun trigger with the other.
For especially hard-to-reach areas, you can use a flexible straw that will bend near one end. And if the straw is too small in diameter, you may be able to just slit one end so it will fit over the nozzle.
Here's another tip for you: This technique works best if you can warm the tube of caulk first. Then the caulk will flow more easily as it moves down the full length of the straw.