To build a snowman in Minecraft, you'll need three snow blocks. Place the first snow block on the ground and then stack two more snow blocks on top of it. Once you've built the snowman's body, add a pumpkin or jack-o'-lantern for the snowman's head. Finally, use coal or emeralds for the snowman's eyes and buttons, and a carrot for the nose. You can also add other decorations like hats, scarves, and brooms to customize your snowman.