Summarize this article in one sentence.
Fill the bag no more than halfway to avoid spills or tears.  Be aware of the areas of your boot that need the most stretching, as this may alter how much water you place in the bag. If it’s a smaller or larger portion, fill the bag with the amount of water that makes it fit comfortably within that space. Use a high-quality plastic bag. You don’t want any water to touch the leather directly, as this will damage your boots. Choosing a bag specifically labeled for freezer use will help ensure that no leaking occurs. Leaving only a small corner of the bag unsealed, remove the excess air from the unfilled half of the bag, and then fully seal the bag. Double check for leaks, tears, or an incomplete seal. Just in case you’ve spilled a bit of water while filling or sealing the bag, wipe it off with a dishcloth or paper towel. Check to make sure that the bag presses against the area(s) of the shoe that need to be stretched.  Use crumpled newspaper to keep the bag in position. You can use several bags of varying sizes to fill the various portions of your shoe. The toe, in particular, may call for a smaller bag that fully touches the entire front of the shoe. Put your boot, complete with filled bag(s), into your freezer. Leave the boots in the freezer for a minimum of eight hours, or overnight. The water should be completely frozen before you remove the shoes, as the expanding ice is what stretches the leather. Once you remove your boots from the freezer, wait at least twenty minutes and up to an hour before you remove the bags. The ice needs time to melt and soften a bit so that you’re not forcing the bag out of the shoe. Taking the bag out too early may tear the bag, leaking water that will damage your boot. Be very careful not to tear the bag as you pull it out of your boot. If your boots are still too tight, repeat the process, focusing your attention on the problem areas.

Summary:
Pour water into a resealable freezer bag. Seal the bag. Place the bag in your shoe. Freeze the boot. Let the boots thaw. Remove the bag. Check the fit.