This guide still works but unit sizes changed since it was first written.
Essentially, you'll start with units weighted towards fighters first to clear air defenses. You ideally want just enough bombers to absorb losses. Try something like 400 fighters/370 bombers to clear interceptors and test. If you lose few bombers, you can add fighters to minimize losses. If you lose fighters you might need to add bombers to absorb more losses. You can remake your attacking unit after each attack, again, to minimize what you are losing.
After air defenses are cleared, you want to make a unit with max bombers to hit targets. 524 bombers I believe.
I just took my first C3 with my new account, and I'm in the mist of taking a second. Unfortunately, I plan on keeping these countries and I am breaking a rule in the guide that is costing me a bit.. If you attack an adjacent country with air, there is a malice to attacks if there isn't a land unit near your target. This is forgiven if you don't share a border.
Edit
Someone posted the following as an example of the guide not working:
Terra Povra (the attacked country,C3WL2) reports:
The country lost 25 missile interceptor batteries, 12 defensive missile batteries and 300 interceptors.
The country used 645 missile interceptors in the defense.
532 soldiers were killed and 1547 were wounded.
The defence was assisted by Defense Interceptor Wing Oildale and Garrison TG5779.
The attack added 0.04 damage points. The War Index is now 56.61
Letiland (the attacker,WL1) reports:
Fighters01 lost 120 precision bombers and 35 fighter planes.
Fighters01 lost trucks, gasoline and military supplies.
The attack may have been reduced or eliminated by Anti Missile Missiles.
755 soldiers were killed and 2165 were wounded.
The War Index remains 53.30
This is actually an example of a pretty good attack, 300 defending interceptors were destroyed for the loss of 120 bombers and 35 fighter planes. You might be able to optimize the unit just slightly, add just a few more bombers, and you might end up losing something like 125-130 bombers and no fighters but, overall, this is an excellent result in an attack to clear air defenses.