Board Thread:Questions and answers/@comment-43770328-20191117233821/@comment-27306930-20191118000834

You just need a simple soldering iron. Here's a basic one from Amazon and here's one from Walmart.

Using a sharp knife, strip an 8th inch of the insulation off the red wire, then apply a tiny drop of solder to the end of the wire. That's called "tinning" the wire. Take out the batteries, then warm the battery terminal. When it's liquid, touch the tinned red wire to the terminal and remove the iron. Wait two seconds or until your fingers burn, whichever comes first 😮

Button it up, you're done !

EDIT:  The one from Amazon is probably a better deal because it comes with some rosin core solder. You'll have to buy that separately with the Walmart one.