Go through this guide and follow the step-by-step instructions to make a Secret Entrance.
In order to make one, you have to place three oak planks on the ground first and add an item frame on the middle block. Then add an iron pickaxe on this item frame.
After this, place two more oak planks at the back and a redstone comparator at its top. Use redstone dust on the next one.
Place a piston on the left side and use redstone dust to connect it with the comparator.
Now you have to cover the redstone by placing oak planks on the front side. And place a furnace and crafting table as shown in the image below.
Rotate the iron pickaxe on the item frame to push the crafting table backward.
The next thing you need to do is to dig ground up to 3 blocks deeper.
This is your Secret Entrance, push the crafting table forward again to cover it.