Just a warning this is by far the hardest and fiddliest step of them all.
First off you must cut the seal off the bad about 2cm (half an inch for simplicity's sake) below the seal (see picture). Then put the seal inside the bag to see if its the right fit (it should be, see step 3). You may have to make small adjustments by pinching and taping the sides of the bag. If it does seem to fit then cut the sides of the seal from the bottom to under the actual seal (so it fits in most bag shapes).
The next part is the ordeal, first, tape down one side to the edge with small scotch tape (which will be removed) and either glue or tape the other side of the seal to the bag, making sure the plastic above the seal is aligned with the edge of the hole.
I had little luck with glue but found that taping the plastic (not the seal) next to the seal to the bag was effective and strong. If you are successful you should be able to seal the bag (its just as difficult as it was when it was part of the plastic bag).