Per the Hot-Shot SABRE-SIX Electric Livestock Prod Replacement Handle manufacturer, remove the end cap, place all batteries with the positive end facing down into the unit, negative side facing towards the open end, and replace end cap.
To install the batteries in the Hot-Shot SABRE-SIX Electric Livestock Prod Replacement Handle install where the end cap is removed for placement. Please find the guide for a picture in the document.