To use the Arm Attachment, you need to already own the Flowpression Boots Pro+, as the Arm Attachment is an accessory and does not come with its own pump.
The Arm Attachment has one hose connector, which means you need to block the second outlet on the pump using the cap located on the underside of your pump. Then, connect the hose to the active outlet, select “Arm” in the pump menu, and start the program. After that, you’re ready to begin your session!