In Spider Spawner Close to Spawn seed, the spawn is in a birch forest next to a cave. Turn to your right and find a way down there.
Enter the cave. It’s very dark down there so make sure you have some torches which you can place down on the ground or on the walls to light it up a little. Close to the entrance you should be seeing the red eyes of a spider further away and that’s the sign that you are close to the spawner and just need to continue straight ahead for a bit further into the cave.
Both of the chests have decent stuff so make sure to grab that while you are down there.