Prepare the liners. There must be at least a 6-inch overlap between the two. Lay your first liner over the second to 6 inches. Use the chalk to mark where the first liner ends on the second liner. Fold the edge of the first liner 6 inches back, so you can easily apply the EPDM adhesive to the underside.
Put on a pair of safety goggles and solvent gloves. EPDM glue is hazardous, and you should take precautions when working with it.
Mix the EPDM adhesive according to the manufacturer's instructions. Some adhesives will come with a wooden stick to use for this. If your adhesive did not come with a stirrer, use a paint stirrer, the type sold at hardware stores.
Using a paintbrush or roller, apply the EPDM adhesive to both liners. Apply the adhesive to the folded back portion of the first liner and to the portion of the second liner outlined by chalk in Step 1.
Allow the adhesive to dry and become tacky, according to manufacturer's instructions.
Start at the bottom of where the two liners will come together. Take the folded-over portion of the first liner and flop it over the second liner so that the parts with adhesive come in contact. If this is a large liner, don't try to do the entire seam all at once.
Use the seam roller to roll the seam for a tight fit. Be sure to squeeze out any air bubbles. Allow the liner to dry according to the adhesive manufacturer's instructions.