The other day I had to seal some popcorn and millet in Mylar bags and five-gallon buckets. It is quite easy. Open the Mylar bag and put it in the bucket. Fill the Mylar bag with your product to about two inches from the top of the bucket. You will notice that the bag is a lot taller than the bucket, rather than cut of the excess I seal it right on the edge. If I open the bucket to rotate the food this leaves me plenty of material to reseal the bag.
I use an old two-foot metal hand level and an electric iron to seal the bags. The level is just the right length and thickness to make a good seal. I do not use my wife’s good iron, some Mylar bags leave residue on the iron. I got the cheapest one they had at Walmart. This keeps me out of trouble.
After putting in the oxygen absorbers, I set the level across the top of the bucket and lay the Mylar bag against it. I then iron it to make the seal. Leave a small opening in one corner. This permits you to squeeze out the excess air. You then make a diagonal seal across the corner. Since I get my oxygen absorbers from the LDS cannery and they are the 300CC type, I always put in 5 absorbers. Fold the Mylar Bag into the bucket. Put the lid on the bucket, label it and you are done.