If you mean the daisy rein as shown on the webpage........
basically you just unfasten the buckle where it attaches to the headpiece of the bridle.
It seems odd that the daisy rein would be left attached to the bridle rather than the saddle. To leave the daisy rein attached to the bridle means undoing two buckles at the saddle end but to leave it attached to the saddle means undoing only one buckle at the ridle end as well as not getting in the way for putting the bridle back on.
I'm sure if this is at camp then you will have lots of help and noone will expect you to deal with tack you are unfamiliar with