The installation process may vary depending on where you intend on fixing the foot hammock.
Office foot hammocks for desks usually require you to bolt a clamp onto the underside of the desk or fix a strap that could engage the desk in some way. After the initial hardware is installed, subsequent use is fairly simple.
In the instance of foot hammocks used by frequent fliers, the strap hooks around the tray table so no additional fix points are required. In cars, it could hook around the headrest of the seat ahead.