Here are some things to consider:
It's always better to cooperate on a cast out rather than try do the whole thing yourself, this game is about diplomacy, if you arrange for other players to cast certain BB's for you in return for something or even just to help end the game quicker, Thats fine.

You may have a few naps that are doing well around you, offer these players the chance to close out the game while they're all doing well, maximise the return for those involved.

Another option is all the players of one religion joining forces to cast out & so get the religion bonus, These are good valid manouvres.

Everygame is different & so presents it's opportunity in different ways from game to game but basically, you'll mostly work with trusted Naps to cast out or join a religion alliance to do so, these are the 2 main methods of casting out a game.

Basically, all you need do is select one player as the "Mule"
Other players then cast BB's along his border & he collects them.

in the case of the Frozen sea & Mtns, you'll need a form that can traverse these terrains to take it as it's cast.
Other option is to have player cast the mountain BB, then the "Mule" Grunches onto it the following tick.

Unless your creaming the field & don't need help, the easiest way to cast out is to work with others to acheive it.
Either way, by about tick 20 you should certainly begin looking toward the end.

have the ravages of war weighed heavily upon your empire? or have you conquered & so can give more toward the cause, Thus is the role of the pen as mighty as the sword in deciding how these wars shall end.