You don't. Inputs are Mono only. You need to use an instrument cable instead.
You'll need 2, one for Left and one for Right if you're recording stereo signals, although many keyboards have only one output which is mono. Check your keyboard specifications for the output jacks are configured as.
Insert cables are used for inserting external effects processors into the signal path of your input, where Tip is the Send signal and Ring is the Return signal.