This tutorial will give you the details to make a mob spawner without lifting a finger, nor it failing, as well as getting the mob's drops.

1. Find a spawner then neutralize it.
2. Have the spawner in a 7(length) x 7(width) x 4(height) chamber (The average size of a dungeon) in the middle.
3. Replace the floor underneath the spawner with Iron Bars.
4. Make a room under the Iron Bar floor you can access.
5. Carefully start putting water in the spawner room without flooding the torches.
6. Once you've covered the room with fairly enough water, run like hell out of the room before mobs start appearing.
7. Seal the chamber.
8. At this point, you should be done. The mobs will eventually drown if you placed the water to cover the corners, above the spawner, and more. They won't be able to get out of the room through the grates in the bars, as they will helplessly try to swim, but they will still drown. The spoils will drop through the grates assuming the room is sealed as the way I explained. (The image should give you a sizable length to your trap.) By the way, don't jump into the bars or the mobs will surely get you.

Diamond if this helped! If this didn't work, tell me in the comments what your problem was.