Most of us have gone to www.serebii.net lots of nice information there. this is just to be a quick run down on special circumstances for obtaining and evolving warlords.
in the primary story warlords in Nobunaga's army cannot be recruited this is primarily because on cannot recruit in a battle that does not result in the defeat of a nation. meaning you must eliminate a nation to recruit the people inside it.
Also no warlord outside the MC may be evolved in the main story.
Now as to the recruitment of warlords the same 3 basic methods apply to them as to the normal warriors with one added the action must be done by a fellow warlord.
in other words one must fulfill 1 of the 3 following conditions
1. defeat in under 4 turns (in a standard 20 turn battle you must beat them by the end of turn 17)
2. defeat them with a super effective attack
3. defeat them before they deal any damage(this makes obtaining gracia possibly the easiest in the game)
one of which must be done with the killing blow being a warlord.
Now evolving warlords, all warlords need to have their perfect link pokemon with a requisite link of about 60% sometimes 5% less or up to 20% more.
Muneshige/Ginchiyo keep them in separate nations farther is better.
Hanbei/Kanbei keep them together at any given time
Ranmaru/Tadakatsu/Kanetsugu/Yukimura they need to be paired with their leader and in the case of Tadakatsu he also needs Ina.
Noh/Nene they need in the respective order all ghost and poison pokemon in the gallery
Kenshin/Shingen you must complete the rival's story
Masamune you must complete his story
finally make sure you take an evolved warlord into battle at least once to register it with the gallery, if you do not do this you risk losing the evolution after the end of the scenario.
other people feel free to add to the tips here :)
my psn: snakbar7 feel free to add me pokemon W FC: 3439-2994-1803