Not too difficult but you need to work relatively quickly.  Use a paint brush to put a coat of slightly thinned white glue on the floor (sometimes best to do in parts - the glue will lose its tackiness pretty quickly).  Distribute the flocking - I use a tea-infuser to sprinkle it on - lightly press down with finger.  Dump excess flocking and repeat.  If you use a flocking pump, you might want some breathing protection.