First, you place down the sacrificial focus.

Then, on the 4 blocks around it you place "Altar" blocks.

Above the altar blocks, you place medium bone blocks.

Above the medium bone blocks you place small bone blocks.

Put 1 of each kind of BoP gem into the altar blocks.

Right click the sacrificial focus with a ghastly sou'l manipulator. Lightning should strike and the texture will change.

Somehow get a villager onto the sacrificial focus. Lightning will strike again.

Right click the sacrificial focus with another ghastly soul manipulator. Lightning will strike, the whole altar will explode and the Ender Dragon will be summoned.

java.lang.NoClassDefFoundError: cpw/mods/fml/client/CustomModLoadingErrorDisplayException
        at biomesoplenty.blocks.BlockBOPGlass.func_71903_a(BlockBOPGlass.java:149)
        at net.minecraft.item.ItemInWorldManager.func_73078_a(ItemInWorldManager.java:575)
        at net.minecraft.network.NetServerHandler.func_72472_a(NetServerHandler.java:874)
        at net.minecraft.network.packet.Packet15Place.func_73279_a(SourceFile:58)
        at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:442)
        at net.minecraft.network.NetServerHandler.func_72570_d(NetServerHandler.java:222)
        at net.minecraft.network.NetworkListenThread.func_71747_b(NetworkListenThread.java:53)
        at net.minecraft.server.dedicated.DedicatedServerListenThread.func_71747_b(SourceFile:30)
        at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:847)
        at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:320)
        at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:716)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:600)
        at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
Caused by: java.lang.ClassNotFoundException: cpw.mods.fml.client.CustomModLoadingErrorDisplayException
        at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:273)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        ... 13 more
Caused by: java.lang.RuntimeException: Attempted to load class cpw/mods/fml/client/CustomModLoadingErrorDisplayException for invalid side SERVER
        at cpw.mods.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:50)
        at cpw.mods.fml.relauncher.RelaunchClassLoader.runTransformers(RelaunchClassLoader.java:387)
        at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:260)
        ... 15 more