Row cover is used for a few different purposes. The two main ones are to protect plants, especially young plants, from harsh weather, and to protect plants from pests. When to use row cover is a matter of judgment. Definitely use it at the beginning of the season when there is still a threat of frost or even snow to cover newly planted seedlings, and to protect susceptible plants from pests. (For example, covering the summer squash until it matures to protect it from squash bugs is a good idea.) Whenever it is used, it is very important to continue to check on the plants and the state of the weeds under the cover so things do not get out of hand.
To use the row cover, space the mettle hoops every 5ft along the bed, making sure they are firmly pressed into the ground on either side of the bed. Next, unroll a length of the white row cover material over the hoops, using the blue lettering as a guide of where the center is. Secure the material with a red stake at and between each hoop. By catching up any excess material with the stakes, the row cover is less likely to be blown away by the wind. Do not forget to effectively cover the ends of the rows as well.
If any of the row cover is badly mouse-chewed, it should not be used, and does not need to be kept. And when done with the row cover for the season, it should be carefully stored rolled up so it will not be chewed.