For top-load juice pack cases (with a removable top cap):
(1) Separate the juice pack case sections.
(2) Slide your iPhone all the way into the bottom section of the juice pack case so that the juice pack case’s Lightning connector is fully inserted into the phone.
(3) Slide the top section of the juice pack case all the way over the exposed top of the iPhone so that it snaps into the bottom section of the case.
If you've got a front-load juice pack case (with a removable bezel, as found on the Samsung Galaxy S7® or juice pack wireless for the iPhone 6 Plus®, Click to view):
(1) While holding down the back of the juice pack case, pull up on the front corner with your thumb to separate the case sections.
(2) Keeping it flat against the case, slide your iPhone® straight down onto the Lightning connector in the bottom of the juice pack case back.
(3) Fit the juice pack case front section onto the back section.
(4) Press the two juice pack case halves together at all four corners. The case will audibly snap at each corner when it is successfully closed.
The H2PRO for the iPhone 6s/6 and 6s/6 Plus is also a front-load design. Watch this super handy how-to video here.
From there, when you slide your phone into the juice pack case, a connector at the base of the case plugs into the port at the bottom of the phone. Flip the on/off switch of the side of the case (for some models) or press the status button on the back of the case for 3 seconds to start charging your phone. Flip the switch or hold the button again to stop charging and put it back in stand-by mode.