Complications are basically shortcuts you can use to navigate to apps faster on your Apple Watch. Setting up the Runkeeper Complication is not only easy, but will make starting an activity from your watch that much more convenient!
First, force press the main watch face screen. From there, you'll want to select the Customize option.
This will take you to the Complication's display page, where you can customize those options. Tap on the location where you want the Runkeeper Complication to be accessible, and scroll through the options using the crown button on the watch. After landing on the Runkeeper Complication, just tap on the crown button twice, and you'll be returned to the main watch screen again.
After that, the option will be set on the main watch face, and no more crown presses or wondering through the available app's screen will be necessary to start a run. From now on if you want to access the Runkeeper Watch App, just tap on the Runkeeper Complication, and you'll be taken directly into the app to begin an activity!