Pre-made drapery panels are the answer to many window covering dilemmas, but they are rarely the correct size for the window. Joining the panels without sewing is not difficult if you acquire a bit of knowledge and a few tools.
Lay one panel flat on the work surface, one long edge toward you, the bulk of the panel away from you, the top of the panel to your right, and the bottom to your left.
Draw a line, with the self-erasing marking pen, 1/2 inch from the long edge closest to you, from the top of the panel to the bottom.
Place one edge of the iron-on fusible webbing, paper side up, along the marked line on the long edge of the panel; the width of the webbing lays between the marked line and the edge of the panel nearest you. Iron the webbing per the directions on the webbing package.
Remove the paper backing from the webbing.
Lay the second panel, right side up, wrong side toward the work table, with the top of the panel to your right and the bottom to your left. Position the long edge of the second panel on the marked line of the first panel; the bulk of the second panel will be toward you, with the two panel edges overlapped.
Cover the seam with a pressing cloth and apply steam or dry heat as recommended in the fusible webbing instructions. Do not move the iron back and forth along the fabric, as the motion will distort the seam. Set the iron on the pressing cloth and count to five. Lift it off the fabric and move the iron and pressing cloth along the "seam." Then, reposition the iron on the pressing cloth, count to five and repeat to the end of the seam.
Let the fabric rest to cool and set the seam before moving.
Check the length of the seam for incomplete fusing after the seam has cooled and before moving off the worktable. Re-fuse as necessary.