hi Ahsum,
welcome to permies
an easy set for a seed starting
greenhouse is to use woven wire
fence
bend it lengthwise in a hoop and put the plastic over it
the
fence will support and hold the plastic in shape
some string or wire ties or twist ties can attach the plastic to the fence
if you bend the edges so they lay flat on the ground 4- 6 inches or so
you can then hold the greenhouse down by putting rocks or something
on the flattened part
good luck