The straw is a little less than 1/4" in diameter and 4" long. Cut a notch in the straw as shown below. A wooden end plug is used for the mouth end. Sand the top of it flat as shown. It should fit snugly in the straw with a small gap at the top. Slide the mouth piece dowel into the back end of the straw.
Getting the whistle to make a sound without exerting too much force can take some experimenting. There are three variables that influence it. First, the amount of space (caused by flattening one side of the dowel) in the mouth piece dowel. Too much space and the whistle won't work, too little and it takes too much pressure to make a sound.
The second is the position of the mouth piece dowel in the straw. For now place your finger over the left end of the straw. Try sliding the mouth dowel further inside the straw or move it back towards the end. Make a small adjustment, then try it again. Position it until it starts making a consistent sound.
Third, determine the length of the straw. Bend the straw at a right angle. The shorter the straw, the higher the pitch, the longer the straw, the lower the pitch. Experiment with different lengths until you find one you like. Then tape the straw in place. You could also cut the end of the straw off and just place a finger over the end when you blow.
This same process can be used to make a whistle out of wood.
You can also make a whistle with nothing more than a blade of grass. Take a blade of grass about 4 inches long and quarter to half an inch wide. You can tear it in half the long way if you want. Take the smaller half (the one without the center vein), hold it between your thumbs as shown in the photo stretching it tight, and blow gently. It takes some practice to stretch the blade tight so that it makes a good, loud sound. You will be amazed at how loud it can be!