You need to take each quantity in the original recipe and multiply by 1/2.
(1/2 teaspoon)(1/2)= 1/4 teaspoon
(3 teaspoons)(1/2) ... just multiply 3 times 1/2 and the answer is the number of teaspoons of cinnamon.
(3 1/2 cups flour)(1/2) .... change 3 1/2 into a single fraction and then multiply by 1/2 and that will gives you the number of cups of flour.
Good luck,
Jason