HOW to make a plugin PLACE/GENERATE a block?

Okay, so I'm trying to make a crazy plugin.

The general idea is this: When a player places a special block (a Portal, in this case), my plugin will place wood (or any other material) blocks in a specific pattern, in order to suffocate and kill the player.

Sounds pretty strange. But, if I can get it to work, it will provide some serious background for Bukkit plugin development, because it requires a lot of calls to Bukkit/Player methods.

My only issue: How do I make the plugin place a block? When the plugin detects the portal is placed, it needs to place blocks in a to-be-determined pattern until it kills the player... But, I can't even get started if I can't place any blocks!

Any luck?
If you can help, I'll give you all future versions, and access to my server, even though I doubt any of you want it.

TL;DR- I'm making a crazy plugin that, when activated, places blocks until it kills the player, but I can't figure out how to make plugins place blocks in any way.