To capture the plans, you need:
- To clear the building plan village of all defence
- To destroy the Treasure Chamber in that village
- To attack the building plan village with your Hero
- To have a level 10 Treasure Chamber in the village you send your hero from
Typically, players land their hero shortly after they clear the village of defence. It is possible to land on the same second provided the hero is sent after the clearer. Alternatively, if the village holding the clearing wave has the requisite Treasure Chamber, the hero can be sent with that wave.
One set of building plans is neccessary to build a Wonder to level 50. To build further, the alliance must hold at least two plans. Often players in friendly alliances will leave their regular alliance and join with another player with one set of building plans, to eliminate spying problems whilst ensuring both have two copies.
It is very hard to capture plans from another player, because it is easy for them to move the plans. Players will generally not defend the plans very heavily, preferring to catapult their own treasure chamber and capture the plans from themselves with their hero from another village with a level 10 treasure chamber. Good players often build several villages in a cluster, those with plans can have an appropriate level treasure chamber in each.
An alternative approach which some players take is to join an alliance requiring the plans then purposely get themselves banned (by, for example, insulting the multihunter or admin for that server). The plans remain active and can still be used, even though the player cannot be attacked. This approach may not work because it is abusing the rules, itself against the rules and potentially leaves the player open to deletion. Sometimes players are able to remain banned for a significant proportion of the end game, however.
(Note that you cannot defend a Natar village as your troops will immediately be sent home)