Basically the same as for any Si unit: set the Chart Speed to match your boat speed. Si Range should be 2x to 3x the water depth. I would use the 262kHz Si sonar frequency so that it will cover a larger area. Once you find something interesting you can change to the 455kHz setting, lower the Si Range and make another pass closer to the object. Set the Si Sensitivity just high enough so that the white sonar returns are not so bright that they overpower weaker sonar returns.
Be sure and update the software in your 987c Si unit of you are not running version 4.770 in it now.