There is an excellet description in the DIY section at the top of the page! Don't know if the E420 brakes are identical, but they should be very very similar to the other W124 brakes with floating calipers.
This isn't a bad job at all unless the rotors are worn out, in which case you must remove the caliers and swap the rotors.
The quick and easy way to tell is to check for a dsitinct lip at the outer edge of the rotor where the pads don't cover. If there is one, it means the rotors are too thin and need to be replaced as well. They are held on by the caliper and pads and a sincel small screw with the wheel off. Just take the screw out (may need some persuasion) and remove the two caliper bolts (very tight, use a hammer on the breaker bar), put blue locktite on the bolts when you re-install.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!