I have a YC in my notebook - top left shirt pocket, and a YC in the front Left shorts pocket.
If it's an incident where I need to slow things down and let things cool off (most of them) I call the player over, stop the watch, and take out the notebook explaining why he is being cautioned. Check his number and name, and slip the card from the notebook, hold it up, put it back and write the details in. Take a deep breath and get on with it.
If it's a quick yellow it comes from the shorts, shown and put back, quick note in book. If it's a 2nd yellow it's also from the shorts - red is in right shorts pocket so a 'quick left/right' and off he goes.