Start with an adult sized shoebox and open it open flat. Print out the templates for the unicorn head and tail and trace onto a piece of the cardboard.
Add 2 - 3 tabs at the bottom so that you'll be able to insert them into the box for stability. Cut out.
Trace lines on the top of the box where you are going to insert the tabs.
Pierce the end of the box with a craft knife where you are going to insert the head and then insert to make sure it fits. Remove the unicorn head again.
Tape the entire box with white Duck Tape, including the lid. Make sure to use your craft knife to cut the lines to insert the head again.
Place the unicorn head down on your craft mat and tape over with white Duck Tape. Flip over and cover the other side with Duck Tape too.
Trim the head out with a pair of scissors.
Draw a face on the unicorn head with a Sharpie. Let dry (so it doesn't smear) and then flip over and repeat on the other side.
Cover the horn with Mirror Duck Tape (or gold or silver) and trim.
Use the remaining colors of Duck Tape on the mane and tail. Lay a piece of Duck Tape (about 7 1/2" - 8") down on the craft mat and cut a 1" slit in the middle at one end.
Peel the tape up and fold it over but DO NOT close the end with the slits.
Use those two flaps to attach the mane piece to the head and trim carefully with a craft knife or scissors.
Use scissors to cut a wave into that piece of tape so it looks more like horse hair . . . and repeat. Finish the mane that way.
One you are done, insert the head into the box. Open the box and fold the tabs opposite ways.
Use the tail template to cut the tail piece out of cardboard. You'll want to make a tab here too.
Cover the tail pieces with 1" thick pieces of Duck Tape. Trim out the tail with scissors.
Cut slits into the back end of the box and insert the tail piece.
Cut a slit into the top of the box. This is where the valentine cards will be inserted. You can line this opening with pink Duck Tape if you like.