Attach shade cover clips every 4 inches on the two long edges of the shade cover cloth, if the cloth doesn't have grommets along the edge. Count the resulting number of clips or grommets. This is the number of twine pieces you will need.
Measure the distance from the edge of the greenhouse roof to the ground. Multiply that number by two. For example, if your greenhouse roof edge is 5 feet from the ground, your result would be 10 feet. Cut the appropriate number of twine pieces to the measurement.
Set up a ladder next to the greenhouse. Climb the ladder and toss the shade cover over the roof. Adjust the cover so it sits correctly over the roof, with the two long edges along the longest edges of the greenhouse.
String the twine through each clip or grommet along both edges of the cover. Pull the twine through, so the two loose ends touch the ground.
Insert a garden stake into the ground below each clip or grommet. Tie the loose ends of the twine to the garden stakes to secure the cover in place.