Firstly a light spray of water with a very small amount of detergent in it would also help dampen the area before applying the glue.
The glue can be applied from a eye dropper papet, a tea spoon, or even sprayed on but be sure not to move the ballast while adding the glue.
When first put on the area looks just like milk has been spilt over it but after an overnight drying period the glue should have gone hard and become transparent.
The next operation after the glue has gone hard is to vacuum up the complete area to ensure any loose gravel is removed from the track.