Open the crafting table
Place 6 matching wooden planks in 2 vertical columns of 3
Take the crafted wooden door
Place the door on the ground where you want the entrance
Use 3 iron ingots in 2 vertical columns of 3 to craft an iron door
Take the crafted iron door
Place the iron door on the ground where you want the entrance
Use a button, lever, pressure plate, or redstone to open an iron door