With the boat out of the water, no AC power.
1) Remove the cooling water filter/strainer cup. Catch any water than drains. Leave bucket in place.
2) On the top of the AC unit, un-clamp and remove the cooling water exit hose that goes to the above waterline thru-hull. Catch any water that drains.
3) Slip a length of soft hose onto the barb where the hose was attached.
4) Hold hose up and with a funnel, pour antifreeze in
5) Continue 4) until liquid coming out of open filter is darkly the color of the antifreeze.
6) You're done. You can also put the filter back together and open the seacock to dump the water.