Using the supplied LDG cable attached the red end (radio) in to the left most mic jack input on the back of the radio, then attach the black end of the cable (tuner) to the Radio port on the back of the tuner.
If you want the tuner to control the radio then update the Tuner Select property to LAMP (Linear Amplifier) in the FT-891 deep settings. Now when you press the tune button on the tuner it will lower the power and send a CW carrier to tune the frequency.
If you don't have the cable to the tuner it will still try to auto tune when you transmit but it's sub-optimal. So you'll need to drop the radio's power down (below 35W, if you don't you'll ruin the tuner) and change the mode to CW by long pressing the BAND button. Then PTT and the tuner will perform a proper tune.