Sometimes you can't get the window of your car up or down. Sometimes the door will not open when you pull the handle. Then you know it's time to take off the inner door panel.
Steps

Step 1. Open the door
Step 2. If the lock protrudes from the top of the inner panel, remove it--usually by unscrewing it

Step 3. Locate the inner door handle that opens the door
Pull it to see if there is a screw under the handle. Remove the screw and peel off the hard plastic cover around the door handle.

Step 4. Look under the armrest
You will see screws holding the armrest to the door. (Sometimes these screws are under plastic covers that you have to pull out with a flat head screwdriver.) Remove the screws. Remove the armrest. If you have power windows, disconnect the wires attached to the armrest by squeezing the plastic sides of the connectors.

Step 5. Remove the window handle (if your windows are not electric)
Sometimes there is a screw in the middle of the handle under a decorative cover (old VW Beetle). Pry the cover and unscrew. Sometimes there is a retaining ring around the bottom of the handle. Loosen the retaining ring from the handle with a flat head screwdriver.

Step 6. Use a wide, flat putty knife to pry the bottom of the panel off the metal part of the door
The panel is attached to the metal part of the door by means of several plastic grommets, which are attached to the back of the cardboard panel. Gently push the grommets out of the holes they are in, being careful not to pull them out of the cardboard panel.

Step 7. Check for screws near the rear view mirror or on either side of the windowsill (Audi)
Remove any screws.

Step 8. Lift the window sill out of the slot, and pull the panel away from the door

Step 9. Gently pull the plastic away from the door so you can see what needs to be repaired
Tips
- Some cars use a Phillips screwdriver, others use hex keys, and some use a torx screwdriver.
- Every car manufacturer is different, so some things you may have to figure out for yourself. You may be able to find images on the internet that will help.
- Stick the plastic back on. It's tempting to just hang it in there.
- Parts for windows are often available on Marktplaats.