Cut the tube into two bits, one 50cm long, the other 25cm long. Cover one end of the short tube with the scrap of muslin and secure it with the elastic band. (This will stop you sucking up the bug.)
Cut the card to the shape of the top of the jar and then cut two holes in it to push the tubes through.
Tape the card firmly to the top of the jar and push the ends of the tubes into it. Fix the tubes in place with the plasticine or blu-tak, making sure any gaps are sealed.
Hey presto! A brilliant bug-catcher.