Measure the circumference of the lower-most area of the gazebo that will be shaded by the bottom edge of the new cover using a tape measure. Use a partner to help you wrap the tape measure around the structure. If the structure is quite large, you may need to measure a section at a time, then add the results together. Record this as the circumference of the gazebo. If the gazebo has a rectangular footprint, measure the width of two adjacent sides, and record the numbers as Side 1 and Side 2.
Measure the distance from the lowest point to be covered by the bottom of the gazebo cover up to the point where the roof slant begins. Record this as the lower flap measurement.
Measure one section of the roof: Hold the end of the tape measure at a bottom corner of the roof, and measure across, parallel to the gazebo foundation or the ground, to the next bottom corner of the roof. Record this as the roof base. If the gazebo has a square footprint, you won't need to measure another side. If it has a rectangular footprint, measure the next section as you did the previous, and record the measurement as Roof Base 2.
Cut a section of paper from the paper roll the length of the roof base. Using a step ladder if necessary to easily reach the roof, align the section of paper along the length of the roof base and secure it in place with masking tape. The paper section should rest against the roof of the gazebo, and the straight bottom edge of the paper should line up with the bottom edge of the roof line.
Fold the ends of the paper so that they conform to the narrowing sides of the roof. If the paper isn't wide enough to reach all the way to the peak of the roof, tape additional lengths of paper to it until it reaches the roof peak. Continue to fold the paper sides until you've reached the roof peak. Carefully remove the tape holding the paper to the bottom roof line, then take the paper off the roof. With a scissors, cut along the folds: This is your pattern piece for the roof section of the gazebo cover. If the gazebo has a square footprint, you won't need to make another pattern. If it is rectangular, you'll need to repeat this step along the next expanse of roof line to make a pattern piece for the next section of the roof line as well. Label the roof pattern piece(s) and set them aside.
Count the sides of the gazebo to establish whether it has four, six or eight sides. Record this as "gazebo sides."
Draw and cut the lower flap pattern. If the gazebo has a square, six-sided, or octagonal footprint, establish the required width of each lower flap section by dividing the total circumference measurement by the number of walls. Add 1 inch to this number, then cut a section of roll paper to this length. If the gazebo has a rectangular footprint, cut two lengths of roll paper, one the length of Side 1 plus 1 inch, the other the length of Side 2 plus 1 inch.
Line up the bottom (short leg, or "T" end) of an L or T square so that it is even with the bottom edge and corner of the paper sections. Following the long edge of the square and using it as a straight edge, draw a line at a right angle to the bottom edge of the paper. The line should measure the length you recorded as the lower flap measurement plus 1 inch. Repeat for the other end of the paper length. Use the yard stick as a straight edge, and draw a straight line connecting the ends of both of the previous lines to form a long rectangle. This is the pattern piece for the lower flap of the gazebo cover. If your gazebo has a rectangular footprint, draw the second pattern piece following the same procedure.
Tape together the gazebo side and gazebo flap pattern pieces, folding over a 1/2-inch seam allowance at the hem area and at the seam area, and test the pattern pieces against the roof of the gazebo to ensure a good fit. Adjust the pattern pieces as necessary.
Pin the pattern pieces to the canvas and cut out the canvas pieces. You'll need to cut one side flap piece for each wall of the gazebo, and one roof piece for each section of the roof. Remember to cut two longer and two shorter pieces of both for gazebos with a rectangular footprint. For all other gazebos, the pattern pieces are the same size for each wall or section of roof.
Turn under a 1/2-inch hem along the long edge and both short sides of all cut-out bottom flap pieces of canvas, and sew the hem in place.
With canvas fabric right sides together, stitch two triangular-shaped roof sections of the canvas together at the side seams, allowing a 1/2-inch seam allowance for each seam. If your gazebo has a rectangular footprint, remember to stitch the wider triangular roof piece to a narrower one. Repeat with all roof sections until you have only sewn pairs of roof sections. Then, sew the sections together, right sides together and stitching along the slanted side seams and again allowing a 1/2-inch seam allowance, until you have a tent-shape. Finish all the raw seams by turning them under and edge-stitching or by zigzag stitching over the raw edges to prevent fraying.
Sew a lower flap section to the bottom of a section of the roof piece, with right sides together, and allowing a 1/2-inch seam allowance. Make sure to match the longer lower flap sections to the longer roof sections if your gazebo is rectangular. Finish the seam as described above to prevent fraying. Repeat this step until all lower flap sections are attached to the roof piece.
Iron the gazebo cover, pressing open all seams as you iron.