Roll up a piece of paper into a cone. Trim the bottom of the cone so that it will sit flat on your head. Tape or staple a ribbon or elastic to the bottom of the horn. Tie the horn to your head.  Decorate the horn with markers, crayons, glitter glue or stickers. You can also use a gold or silver party hat to make a horn. Unroll the party hat and cut out a 1-2 inch section from it. Re-roll the hat and tape it into cone shape. Tape or staple an elastic onto the bottom of the hat. Wear a long-sleeved shirt and leggings or pants. Wear white, pink, purple, or another pastel color. Put stickers on your shirt to add decorations. Use curling ribbon or yarn in pastel colors to make a tail. Cut several pieces of ribbon or yarn to reach from your waist to your knees. Tie these pieces together at one end and pin or tie them to the back of your pants. Wear black or brown shoes to stand in for hooves. You can also wear black or brown gloves as your front hooves.

Summary:
Make a unicorn horn. Wear white or pastel colors. Make a tail. Complete the outfit.