Patterns come about when the sun reflects on blades of grass that have been pushed down by a roller on the back of a mower. When the mower goes in one direction, the blades are pushed down in that direction. Go in the opposite direction and the blades will be pushed down in the opposite direction. This alternating pattern creates the light, dark, light, dark pattern. To get stripes or other designs in the lawn, you can buy a device called a lawn striper and attach it to your mower. Fill the lawn striper with sand or dirt and attach it to the mower using brackets that come with it. The extra weight provided by the sand will push the grass blades lower to the ground.
If you don’t want to buy a device to make stripes, you can improvise by using some chain and a welcome mat. Attach the mat with the chain to the lower part of the mower handle. When the mat is dragged behind the mower, it will force the grass to lie lower, creating the appearance of a stripe.