I DO use that flocking kit with very good results.
Finish everything first. I use a plastic container for the entire kit. Easy to contain the mess and store the stuff. Put on enough colored glue, double check. Put the box IN the plastic container and flock. DO NOT SHAKE THE BOX. Carefully remove the box and check for any shiny areas, these don’t have enough flocking. Reflock the box until there are no shiny spots. Extra flock is fine. Let set at least overnight. Turn the box over and shake the extra flock into the plastic container. This can now be put back into the puffer. After 24 hours you can tap the bottom of the box to remove the rest of the unstuck flock. This will take a bit. I usually turn it over upside down and ‘drop’ it about an inch onto the table a few times. The “trick” is letting everything dry.