Step 3 · Apply Silicone Sealant
After confirming the dry fit, remove the extender(s). Load the tube of 100% silicone sealant into the caulking gun. Apply a continuous, generous bead of sealant (about 1/4 to 3/8 inch thick) directly onto the clean top surface of the existing flange. If you are stacking two extenders, apply a similar bead of silicone to the top of the first extender before placing the second one on top.