Grasp the grey slider seal and pull until it slides off of the top of the reservoir. Flip the flap up. Located towards the top of the flap, pull the thin TPU material away from the top in opposite directions. This will open your reservoir and you'll be ready to fill it up.  You can watch a video HERE. *Brand new reservoirs may take more effort to open on the first use. After the reservoir is open for the first time, it will be much easier to open for the rest of the life of the product.*