As stated... if you haven't already done so, remove the engine drain plugs letting all the water/AF out of the block. This is absolutely necessary. your engine block needs to be empty. The AF is just insurance for any little water that may be trapped. Probe the holes to make sure no rust or scaling is blocking them. Put your drain plugs and your bilge drain plug in a zip lock bag taped to your steering wheel. I'm not familiar with your particular engine. You need to find out how many drain plugs there are and make sure you don't miss any.
Pull your thermostat and pour AF down there until it spills out of the block drain holes. You may also need to pour AF down some or maybe all the hoses that attach to your thermostat housing.
You should also change your stern drive gear lube. If water happened to get into there, it will freeze and wreck your drive.
Most of the pros here will tell you the AF is not needed because "air don't freeze". They've been doing this for many years.
I'm just a super cautious guy and like the extra insurance, so I spend a little more and run the AF through it and then drain it.