I've tested it now, and it looks like it's best putting the dry fabric onto card which has been sprayed with glue, rather than spraying both the card and the fabric. See pic*
Top strip of fabric is dry and placed onto sticky card, bottom strip had glue sprayed onto both card and fabric. As you can see, when I sprayed onto fabric the driggles of glue must have seeped into the fabric a bit and thus show through.
But the top strip has no such markings.
I waited only about 1 minute (as long as it took me to get back upstairs (I sprayed outside) and cut a quick new strip of the fabric to press immediately onto the sprayed card.)
Instructions say wait 1 to 2 mins, for glue to dry a little and become sticky rather than wet.
So that worked out quite well!