Firstly Let me stress this, This is not the only Design, there are Many!

Materials Needed:
-2 Water buckets
-10 Torches Approx (5 on Spawner)
-Lots of Cobble, 1-2 Stacks
-Signs 2-3 Stacks
-Pickaxe(s)
-Laddersx20
-Stairs x4 or 8

You can See how to make the Grinder Via Video or follow this:

Quick Steps Guide:
1) Find the spawner (Clearly)
2) DISABLE not DESTROY it
3) Make an Infinite water Source
4) Mine out the Floor by 2 Levels
5) Place water along the Cobble,
6) Find where the water ends its stream, and Mine 2 Rows out
7) Box it in!
8) Mine 14-18 Blocks Down for ALL the Streams, Placing Signs on the wall as You go
9) Make a hitting Bunker, by mining out the Bottom Row along the Back wall, and 2 More layers down
10) Make your way safely up to the Surface with ladders
11) You can Put light-lock windows in
12) Enjoy your Grinder