When installed, the 009 pads flaps are between 2 metallic disks that are held together by 6 screws and press the pad's leather flap between them. Between the outer metallic plate and the pads inner surface are the grills.
For disassembling the pads, first you need to get the cloth grill screen out. It has a reinforcement ring on the outer perimeter, and the cloth is tensioned on that. Just put your finger under the pad until you feel the edge of the grill, then bend you finger to bend the grill, until it comes out. It will bend in about 90 degrees but it's quite flexible and you can restore it later, many times.
When the grill is out, you see the metallic grid. You can try the headphones without the cloth grill. It will sound more open and more clear in the treble, but quantity will be too much.
With a finger slightly lift and pull away the pads. That will reveal the screws that hold the metallic disks together. Loosen the screws so that the disks come 2-4 mm apart.
Now put your finger under the pad, and pull it out until the circular flap comes out from between the two disks. Do that on the circumference and the pad is off.
After the pad is off, and you want to experiment with different pads, secure the screws again temporarily so that the 2 disks are not loose any more. For testing, it is enough to change the pads and then just press them against the driver and carefully put it on your head. It does sound slightly different when the pad is fully assembled, but it's really close and much better for testing than to assemble/disassemble the pads so many times.
To assemble, do the reverse steps: loosen the screws again so that the disks are 3-4 mm apart, then insert the pads's flap into the gap, going through the circumference. After it's on, pay attention to the orientation of the pads, and the position of the masking plastic strips in the bottom of the drivers where the opening for the cable is (you will see on the spot what I am talking about). After everything is in position, secure the screws, starting with one but don't tighten it yet, then secure the opposite side, and so on, jump always to the other side, just like when you are assembling a car wheel. This is to get the pressure even.
After the pad is on, reinsert the grill cloth. This is the most tricky part
. I usually lift the pads with one hand, and push one edge of the grill in place. Then I bend the grill and push the edge which is about 90-120 degrees from the previous spot. The remaining edge is badly bent by this time, but don't be afraid of just pushing it into place. It slides smoothly on the metallic grill to go under the pads. After it's pushed in, go around the circumference of the grill and even out the plastic reinforcement.