Place the protractor on your piece of wood. Be sure to line the straight line on the bottom on the protractor with the bottom of the piece of wood. Align the small hole on the bottom of the protractor with where you want to begin your cut.
Mark a dot with your pencil in the small hole on the bottom of the protractor.
Find 45 degrees on the rounded part of the protractor and with the pencil, mark that spot on the wood.
Remove the protractor.
Align the dot from the small hole in the protractor and the mark signifying 45 degrees by using a ruler.
Use your pencil to draw a straight line with the ruler.