Step 2: Get Cutting
Cut the bottle in half (Remember your knife safety). This is also a good time to dry it out. (Remember, you will be putting electronics in here to protect it from water, so it should be dry).
You will also need to cut the cap. If you bottle has a sport/sippy cap, cut out the post holding the mouth piece from the inside. Check to see if the end if the USB cable slides in easily. If not, you will need to carefully cut a slit in the tube on the top of the cap.
If your water bottle has a normal cap, place the knife point in the center of the cap. careful not to cut or stab yourself or anything else, push the knife down, creating a slit. Now, on on the edge across from that slit, make another slit that meets the first slit in the center. In the same manner, continue making slits until you can easily push the end of the USB cable through the cap.
With the USB cable through the cap, screw the cap on to the top of the bottle.