My spacebar is broken, want help remapping it to my Option key
Using Karabiner is fairly simple if what you're trying to accomplish has a built-in remap option. You create a profile for the keyboard you want to remap (from the menu bar icon) and then open Karabiner’s Preferences page. From there go to the “Change Key” tab and select the remaps that you want to include.

If there are no built-in options to swap/remap the space bar to suit your preference (it's been a little while since I've had to use Karabiner so I can't remember the complete list of swaps) there is an extensive guide here which will show you how to create your own key-swap option by editing an .xml file:

https://www.keychatter.com/2014/08/04/how-to-remap-keys-in-osx-using-keyremap4macbook-now-karabiner/ -- (Scroll down to the "Advanced Remapping" Section)

(I included the link only as the full article steps are too long to post in a stackexchange answer)