Apply flap stickers

Applying the letter stickers to each of the 40 flaps is a painstaking, slow process. After a bit of practice, it still took me about 1.5 hours and a lot of patience to apply all the stickers to a new set of flaps:

Note: I've since discovered a better way of applying letter stickers - see New Instructions. The Old Instructions are still included below, but no longer recommended.

New Instructions

Watch the youtube video:

This method is a lot less stressful and offers much better alignment than the older method.

1. Tape down old credit cards or hotel keycards so they form a U-shaped jig that cradles 2 flaps. Place 2 blank flaps into that jig. (Optionally tape another card or piece of paper that covers the bottom 3/16 inch (4.76mm) of the bottom flap -- this serves as a baseline for aligning the stickers)
2. Using food storage cling wrap applied tightly over a clear container, place the letter sticker upside down (sticky side UP) on the cling wrap. It should be attracted to the cling wrap because of static electricity, which will allow us to flip the whole thing over without the sticker falling off.
3. Flip the container+cling wrap+sticker over, and looking through the clear container and cling wrap, align the letter onto the 2 flaps (using the optional baseline alignment guide mentioned above, if you have it). Press the container down so the sticker adheres to the flaps and remove the container.
4. Using your finger, press down all over the letter sticker to ensure it's adhered and there are no air bubbles.
5. Cut the letter sticker in half along the gap between the 2 flaps.
6. Remove the bottom flap from the jig and set aside. Take the top flap and flip down so it's in the place of the bottom flap, and place a new blank flap in the top position.
7. Repeat steps 2-6 with the next letter sticker.

Old Instructions

I've found it's easiest to keep the order correct by starting with all but two of the (empty) flaps already on the spool. You can add/remove flaps on the spool by gently squeezing the flap so the middle of the flap bows away from the spool's center: hold your index finger in a cup shape underneath the flap so only the flap's edges touch your finger, and then press your thumb into the center so the flap bends to follow the contour of your index finger. This will cause the pins to move inward enough to slide the flap into or out of place:

Start with just 2 empty flaps off the spool, flat on your workspace arranged as they would be on the spool (vertically mirroring each other, with the pins in the middle).

For each letter:

1. Use a hobby knife to cut the letter sticker in half. I used a spare PVC card cut to 1.5 inches tall as a guide since it's smaller and easier to move around than a ruler.
2. Peel up half the letter (you can use the knife's blade to help separate the sticker from the backing), and carefully apply it to one of the flaps, with the cut edge along the flap's cut edge.
3. Repeat with the other half of the letter, making sure to align it with the half you already placed.
4. Take the bottom flap and place it back on the spool, in the lower of the two available slots.
5. Take the top flap (still sitting on your workspace) and flip it vertically so it's face-down in the bottom-flap orientation.
6. From the spool, remove the empty flap above the empty slot (above ), and place it on your workspace in the top-flap orientation.
7. Repeat the steps for each letter. You will always be placing the bottom half of the next letter on the back of the flap that has the top half of the previous letter (e.g. the bottom half of "C" is on the back of the flap with the top half of "B", and the bottom half of "9" is on the same physical flap as the top half of "8").