In The Tempest, what plan does Caliban suggest for killing Prospero?

Here's the passage you're referring to: it's from Act 3, Scene 2 of "The Tempest". Caliban is addressing Stephano (and Trinculo): 

Why, as I told thee, ’tis a custom with him,
I'th’ afternoon to sleep. There thou mayst brain him,
Having first seized his books, or with a log,
Batter his skull, or paunch him with a stake,
Or cut his wezand with thy knife. Remember
First to possess his books, for without them
He's but a sot, as I am, nor hath not
One spirit to command—they all do hate him
As rootedly as I. Burn but his books.

Prospero always sleeps in the afternoon - he has a nap, effectively! Caliban wants Stephano to first seize his books, and then "brain" him (batter him on the head) with a log - or, perhaps, stab him in the stomach ("paunch him") with a stake - or, perhaps, cut his windpipe ("wezand") with a knife.

Caliban knows too that the books are the key to Prospero's power, and makes sure that Stephano knows that the books have got to be seized before Prospero is killed.

Hope it helps!