Choose a hidden location for the entrance
Dig a 2-block-high passage behind a wall, floor, or hill
Build a concealed door using pistons
Place sticky pistons to move blocks in and out of the entrance
Connect the pistons to redstone dust
Add a lever, button, pressure plate, or hidden switch as the trigger
Hide the trigger behind a painting, under a carpet, in a tree, or inside a nearby structure
Use blocks that match the surrounding area to disguise the entrance
Add a second layer of blocks or decoration to make it blend in
Test the mechanism to make sure the entrance opens and closes properly
Cover any exposed redstone with blocks
Add lighting inside the secret area if needed
Make sure the entrance is easy for you to access but hard for others to find