Properly wire LED battery level indicator between battery and load
Looked at the link you've provided, scrolled half a page, found this:



Just connect it as it says.

You'd want to connect the V+ to the battery's positive terminal, 3- to the battery's negative terminal.

You can then add a switch to switch both the load on and the indicator; you'd do it like shown on the first picture.

Just use a switch and wiring that can handle your load, and connect the load effectively between the R an 3- terminals. The switch would go as shown between the battery's positive and R terminals.

If you intend to switch a beefier load, just use the schematic from Picture 2, using a beefy enough relay to handle your load.