Use the duct tape to seal any holes that leak light past the cardboard.
Now point the binocular toward the sun while holding a piece of white cardboard about a foot behind the eyepiece. It will take a little effort to find the sun. Once you do, you can focus the binoculars to bring the sun to a sharp image.
Be careful not to put your hand or anything flammable near the eyepiece! The concentrated sunlight exiting there can cause a nasty burn or set something ablaze!