First, you need a data cable for your Lowrance.
Hook the TX line from the data cable to pin 22 on the EFIS or pin 3 on the serial update port. Connect the ground of the data cable to aircraft ground, or any ground on the EFIS.
That's it. Only other issue is to unplug the cable from the Lowrance when doing EFIS firmware updates.
There's drawings of this in the EFIS install guide, on page 2-2 and 3-4.
