Here's a youtube tutorial on this project you can watch it of you don't understand it.
Step 1: solder buzzer
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Step 2: solder battery cap

Step 3: attach battery.

You can attach the battery with the help of double tape.
Step 4: stick on your shirt
I have used double tape to stick on my t shirt.
Now it's ready to use.
After you make it you can stick on your shirt and it's ready to use.
Here's a demo on this project.