Summarize this article:

Hold the longer end of the straw next to your eye and point it at the top of a tall object you want to measure, such as a tree. Most likely, you'll have to tilt the triangle so one of the bases is parallel to the ground in order to see the top of the object you're aiming for. In order to measure the tree, you need to find a spot to stand where you can hold the triangle completely flat and still see the top of the object through the straw. You can tell when the triangle is flat because the weight will pull the string down exactly in line with one of the triangle's short sides.  When this happens, it means the angle of elevation between your eye and the top of the object is 45 degrees. If you crouch or stand on an object to find a better position, you'll need to measure your height at eye level while in that position, instead of when standing normally as described in a later step. Just like the triangle you're holding, the giant triangle formed by you, the base of the tall object, and the top of the object has two 45º angles and one 90º angle. The two shorter sides of a 45-45-90 triangle are always the same length. Measure the distance between the position you were standing in at the end of the last step, and the base of the tall object you are measuring. The result is almost the height of the tall object, but there's one more step to get your final answer. If you don't have a tape measure, walk normally towards the tall object and count how many steps it takes to get there. Later, when you have a ruler, measure the length of one step and multiply by the number of steps you took to find the total distance (and therefore the height of the object). Because you held the clinometer at eye level, you actually measured the height of the object starting at your eye's height above the ground. Use a tape measure to find out how tall you are from the ground to your eye level, add the result to the number you measured in the last step. Now you know the full height of the object!  For example, if your eye level is 5 feet (1.5 m) and the distance between you and a tree are 45 feet (14 m), the total height of the tree is 50 feet (15 m).

Summary:
Sight the top of a tall object through the straw. Move forward or backward until the string lines up with the triangle. Use a tape measure to find the distance between this position and the base of the tall object. Add your height at eye level to get the final answer.