The instructions in your link offer a decent start, but a few details are missing there...
1. All clear polyester film has curvature. Place the two sheets with their convex sides together. This minimizes the "bubble" effect and makes a flatter, better-conforming mount.
2. Instead of laying the tape strips within 3/8" of the document, I suggest no farther than 1/4" from the document's edges, in order to create a pinching action that helps hold the document in place, but no closer than 1/8".
2-1/2. DO NOT USE ATG TAPE, because it would surely fail in a short time. For all clear film mounts, use 3M #889 or 3M #415 double sided tape.
3. Instead of placing the document on the bottom sheet and then placing tape around it, I suggest placing the document under the sheet and placing the tape strips on the sheet while looking through it at the edges. This minimizes the possibility of getting tape adhesive in contact with the document -- which could be disasterous. Be sure you apply tape to the convex side of the clear film sheet.
4. Make sure both sheets of clear film are clean before assembly.
5. After placing tape strips on the bottom sheet, pull their release papers off the corners about 1" only, toward the outside of the mount. Then center the document and the top sheet of clear film (convex side down) on top.
6. Before removing the release paper strips, place a weight in the center of the loosely-assembled mount, in order to prevent shifting of the parts. Again, contact of the document with the tape adhesive could be disasterous. Be careful.
7. Remove the release papers one side at a time and, using the palm of your hand, squeegee the tape bond toward the outside of the mount, being careful not to create wrinkles in the sheets. If you put a document-sized piece of foam center board under your weight, that makes the sheets lay flat, but complicates the tape bonding task a little.
8. Remove the weight and burnish the tape adhesive thoroughly using a burnishing bone. Protect the clear film from abrasion by using a liner of release paper or scrap film under the tool.
9. (Shameless plug with apologies) Buy my new book, "Mounting Objects with Clear Film", from Columba Publishing, the PPFA Bookstore, the PFM Bookstore, M&M, or me. In addition to full details on the Encapsulation Mount, 12 other clear film mounts are detailed to benefit every professional framer.