Just about everyone has had a pair of shoes that just did not quite fit, but that they loved too much to part with. While it is much harder to stretch plastic boots and shoes than it is to stretch leather or canvas ones, you can get fairly good results if you heat the plastic first.
Check your hair dryer's power. You will need a professional-grade hair dryer for stretching your boots because you need to be able to get the plastic warm enough to be pliable. This means that you will need over 90 volts. Your hair dryer will have the volt measurements on the side.
Fill the plastic bag with play dough. You may not need the entire pound, but you do need enough to solidly fill the front and sides of the boot.
Stuff the playdough bag into the boot as tightly as possible. Hold it in place with one hand. If you want your boots wider, then push the playdough into the front and out. If you want them longer, you may need to use the ruler to wedge the playdough lump into the front of the boot.
Use the hair dryer to heat the boot. Focus on the area that you want to stretch. As you are heating the boot, you should continue forcing the playdough forward and outward to get better stretching. Keep an eye on the plastic to make sure it does not start to melt. Do not exceed 10 minutes of heating at a time.
Leave the playdough in place while the boot cools. If you have wedged it in with a ruler, leave the ruler in place also. Let the boot sit for 15 to 20 minutes.
Try your boots on. Plastic is difficult to stretch, and you may need to repeat the process several times before you detect a difference. Wearing the boots a lot to break them in can hasten the process.
Some people use a hot air gun (usually used to dry paint quickly) to stretch plastic boots. This will be far more efficient, but you can melt your boots if you are not careful. Use short blasts of air and monitor your progress closely.