Take a rope (sisal or hemp) of an appropriate length.  Depending on the
sub, situation, etc this could be as short as 5 ft to as long as 50 ft.
The rope is about crotch height.  

The rope is soaked with a flammable fluid, the selection based on how
fast the sub should run.  The fluid might be alcohol, lighter fluid,
kerosene, etc.  There should only be enough to burn reliably, not soaked
for a huge flame.

          ______________________
         /|                    |\
        / |                    | \
       /  |                    |  \


You should have something that looks like that with the rope at top at
crotch height.

The sub starts at one end and has to run past the other with the rope
rubbing her crotch.  The rope is lit behind her so if she runs fast
enough she runs faster then the fire spreads along the rope.

There should be a pool or wading pool for the sub to jump into to
cool off.

There are many options:

    The sub can be wearing panties and optionally a bra with a 'fuse'
    leading from the panties to the bra so if the panties catch fire,
    so does the bra.  A little fluid could be put on the panties and bra
    to insure that a close call will insure they catch fire.  Instead of
    or in addition to panties and bra, other clothing could be worn,
    such as corsets, girdles, hot pants, etc.

    The rope could be lit at the other end so the sub is runing toward
    the advancing flame.

    The rope could be lit at both ends.

    Instead of lighting the rope, the panties are lit and the sub has to
    run down the rope, setting the rope on fire.  In this case, instead
    of coating the whole rope, you might want to coat every other foot
    of rope so the sub has to make sure she lites all the coated areas.