Thank you to everyone who commented, I rushed the writing portion.
I'll try to clarify some steps to make it easier to follow.
Please vote for it too!
A grande size cup
A tall size cup
A razor or knife
A pencil and marker
A hot glue gun
Step 1: Making Candy Hole
2. Use a pencil to mark off a square larger than your candy. This was about an inch for me.
3.Cut out the hole with a knife. Make it a clean cut, we'll save this piece for later
Step 2: Trimming Your Tall Cup
2. Flip it over and cut out the bottom piece, save this too.
3. Tear cup along the seam as cleanly as possible.
4. Now slide this into your grande cup, you'll notice the tall cup overlaps. Mark this new edge (see image) with a pen.
5. Remove the tall cup and reseal it along this edge with hot glue.
6. Put your slenderized tall cup back inside, and trace the candy hole with a pen. I forgot to take a picture of this step, so I made one in paint.
7. Cut out your matching hole, once again save this scrap for later.
Step 3: Making the Sliding Tab
2. Make two lines (about an inch apart) perpendicular to this line.
3. Cut the two vertical lines.
4. Bend this over to create a tab, and crease the fold.
Step 4: Combining the Cups
2. Line up the two holes.
3. Take a pen and mark off the top of the tab, with the hole open.
4. Twist the cups to the side, (either way) until the hole is closed, mark it again.
5. Remove tall size cup. You should now have two little marks
6. Use a knife to connect these two lines into a slit. This was awkward so be careful with knives.
7. Now you should be able to make the slit bigger, slice a second line just under the first. Tear this out.
8. Put the tall cup back in, and slip the tab through the slit.
9. Test it back and forth. I had to trim an edge because it would catch. Look for any issue.
Step 5: Finishing Touches
2. Insert this into the tall size cup, angle it downwards towards the opening.
3. Hot glue this to the tall size cup, it'll be easier to do this with the cups separate.
4. Reassembly the cups, pushing the tab through.
5. Twist the tab into the "closed" position.
6. Take the piece we first cut out from the grande cup. Place it onto the tall cup wear the hole would be. If you don't have a graphic on your cup you could skip this step.
7. For dark cups, take a pencil around the white edges to make it less noticeable.
8. Make sure your sleeve still fits over the cup with the tab there. If not, you can tear it along the seam and re-glue it for a looser fit. Remove the sleeve.
9. Place a dab of hot glue on the outside of the tab, slide the sleeve on over it. Don't push it up too far because you want to be able to move it easily.