Measure the lawn furniture so you have correct measurements. Go to a party supply store and look for vinyl tablecloths that are as close as possible in size to your furniture measurements. You may have to sew together some of the tablecloths to cover large pieces. Clear a workspace on the floor or on a flat surface such as a long table to have plenty of room to lay out the tablecloths.
Open the vinyl tablecloths with the soft cloth side up. Use the soft side to cover wood furniture outdoors. Take a pencil and a ruler and trace the measurements of the furniture for each section. Cut with fabric upholstery scissors the pattern of the furniture. Sew the covers together with strong thread or use a sewing machine and reinforce the seam as you sew. Store away any extra vinyl for patch jobs. If the vinyl tablecloth rips, the vinyl easily can be repaired.
Use plastic shower curtains purchased from a discount store. The curtains are long and wide and not as many are needed to cover furniture. Use the shower curtains on lawn furniture that is outdoors but under cover from the elements.
Sew shower curtains together by hand using strong thread. The curtain may not be as durable as vinyl table covers, but easily can protect well against moisture in the air and light rain. The shower curtain also can be placed over vinyl tablecloths that are protecting furniture from being scratched. Sew the seams tight so no moisture can get in, and reinforce seams, if necessary, on a sewing machine.
Sew tarps together for big pieces of lawn furniture such as sofas or tables. Tarps protect against hail and hard rain and will last longer than vinyl tablecloths or plastic shower curtains.
Use polyethylene plastic sheets to cover lawn furniture. The sheets are surprisingly strong and come in rolls. Take one roll and cover the furniture while unrolling the roll. Wrap the plastic around the furniture, starting with the bottom, and wrap in a circular motion upward, layering the plastic so no water can get in as you work your way to the top of the furniture. Cut off the remaining plastic and sew the plastic shut with heavy thread.