Add a spot of solder to the side of the base, if it doesn't already have some there, where the rim has a nitch in it. This is where you will solder the negative (short) leg of the LED. It may take a bit of time to get the base to take the solder because the surface is not clean. Put the tip on the base and then add the rosin core solder, using the rosin to clean the surface. You only need a small blob of solder, so don't get too carried away.