You'll want to use a heat gun for the melting as it does a much better job than a flame. You want to heat it evenly until adhesive oozes out the ends. Blow up the photo and you can see the adhesive.
Once you connect this to the antenna use a self amalgamating tape to seal the entire connector.
There are many methods to install a PL-259. Get on the net, research them and use what you feel comfortable with, you will get many opinions on this. Over the years I have tried most all of the methods and finally settled on this method for many of my PL-259 terminations in the marine environment.
When swinging from a bosun's chair 40-60 + feet up in the air with a 10 knot breeze I can assure you that getting a good solder joint for the braid is nearly impossible, no matter how good you are at soldering. The tip is easy but the shield, not so much, this a crimp solder connector works very well.
Unless you are very skilled at this, which most won't be, you may ruin a few connectors trying the solder/solder method and may still wind up with a cold solder joint and a failed connection.
The crimp solder method has held up very well for me in the marine environment.
Even with this method I advise buying a couple extra PL-259's and practicing first.
Good luck!