Left Side:

WARNING: This page is about a different car, the 2005 BMW 760Li, 2004 BMW 760i, 2004 BMW 760Li, and 2003 BMW 760Li. However, it is still accessible from the selected car via links, so may be relevant.

Unlock flap (2) with lever (1) and open.

Fig 1: Unlocking Flap

Courtesy of BMW OF NORTH AMERICA, INC.

Snap out flap (1) in direction of arrow and remove.

Fig 2: Removing Flap

Courtesy of BMW OF NORTH AMERICA, INC.

Installation:

Following parts of flap (4) must not be damaged or missing:

Mountings.
Lock.
Screws.

Fig 3: Identifying Mountings, Screws And Lock
Courtesy of BMW OF NORTH AMERICA, INC.