Hello, Supah here
Im gonna show you the steps to making a skeleton or zombie spawner. They are both built the same way, but this method of building does not work on a spider spawner
Also, the reason I am in creative is because I made the model spawner on build (It was a little easier having creative)
Hope it helps!:
Find a spawner.
Place down torches around the spawner
Dig a hole 8-14 blocks deep
Cut out a hole for you to stand in while you are collecting xp. It should be no deeper than 8 blocks back.
Place water in the back of the spawner running towards the collecting area so that XP and drops dont get stuck in the back.
Remove all torches inside the spawner.
Place iron bars so that the zombies/skellies cant reach you.
Add chests and furnaces for storing items (Optional)
Congratulations! You now have a completed Zombie/Skelly Xp Farm!