Follow the manufacturer's instructions to preheat your glue gun. My gun needs five minutes’ warm up time. Place a sheet of scrap paper under the tip, as sometimes a dribble of glue drips on to the work surface. Make sure you have a spare glue stick or two!
When ready, apply a dab of glue to each side of each thread so that it is fixed to the block. Pull the thread tight, then apply blobs of glue.
 

All threads are now anchored with blobs of glue. The actual spine surface may now be covered with glue.
 

This is how it looks when the layer of glue has been applied over the whole of the spine.