The most common sized mylar bags are 1-gallon and 5-gallon. When I repackaged my milk, I used the 1-gallon mylar bags. That way, if I want to use some of my milk, but don't want to use five gallon's worth, the rest of my milk will stay fresh. I purchased twenty 1-gallon bags from amazon with oxygen absorbers for about $13. I only used 10 bags to repackage my 55-gallon bag of milk. If you don't want to have 10 bags left over, consider splitting the purchase with a friend. These mylar bags came with the appropriate sized oxygen absorbers, so you don't have to worry about getting those if you buy these bags. Are you wondering why you need oxygen absorbers? Check out this post by Back Door Survival: What the Heck are Oxygen Absorbers?
Start by labeling the contents and the date on your mylar bags with a sharpie. Then open up the bags and place them on your counter or table. Open your giant powdered milk bag(or whatever you're repackaging) and begin scooping the contents into each mylar bag. Don't overfill the bags or you'll run the risk of not being able to seal them properly.
To seal with a clothing iron, use a straight edge, table edge or 2x4 on the under side of the bag and run the clothing iron on a medium heat setting, over the top of the bag to seal. For a video tutorial, click here.
Storing Food with Mylar Bags.
For long-term storage, it's a good idea to store mylar bags in food grade plastic buckets or bins. This protects the bags from being punctured and prevents rodents or insects from chewing through the mylar. Also, be sure to always store food at room temperature or below.