Tools: Paper, pen, scissors or rotary cutter, ruler, and your sewing machine.
NOTE: You can make a simple insert using scrapbook paper! It's changeable and only involves cutting. No need to spray on glitter, just buy glitter card stock.
Tip: Do not get your insert bulky or it wont fit inside the cup. I found that sewing the insert worked best.
Step 1: You'll want to make your insert template. If you have a venti cup your insert template will look similar to this. If you have a different size you'll just have to mess around until you get it perfect. I took a regular sheet of computer paper, rolled it, wrapped it around the inner part of the cup and drew lines over the excess. I then cut the excess off and made any adjustments needed to properly fit. It may take you several tries. Make 2 templates in case you want to make another later.
Step 2: Start with a few pieces. I cut various fabrics into strips or triangles. I took 2 pieces and sewed them together. As you are building your insert you'll want to cut some excess or cut a straight line to make it easy for the next fabric to attach. Keep doing this until your insert is bigger than your template. Check the fitting inside your cup and adjust.
Step 3: Attach your insert to your template by laying the insert on top of the template and sewing lines or zigzags or swirls all over the two. Be careful not to sew over the same area repeatedly because it will bulk up too much. I attached the template to the insert fabric to give it strength inside the cup. Trim any area that needs trimmed along all 4 sides. Check the fitting inside your cup and adjust.
Here are some examples. Start from the inside and work your way out. You can also start from one side and work toward the other! Let me see what you make!
Purchase your cold cup here: http://www.starbucksstore.com/cold-cup-kit-24-oz/bundleColdCup24oz,default,pd.html?start=1&cgid=drinkware