Step 1: Get Two Large Cardboard Boxes
It doesn't matter what size they are but try and make sure they are big enough for a cat. Make sure both boxes are open on one of the large sides.
Step 2: On One Of The Boxes, Cut Out An Opening For The Cat To Get In
Make sure the opening is big enough for the cat to be able to get in.
Step 3: Put The Boxes Together
Hold the boxes together so the openings of the boxes are together (see pics). Then use Sellotape or masking tape to seal them together.