levinmr,
If you look at the various posts in this forum you'll get a great listing of everything you need to do. I agree that the safest way is to pay a mechanic and watch him, however, this really isn't a complicated process. Just make sure you follow all of the steps. I also have a Wellcraft Martinique 2600 and do my own.
Engine - You will see 2 different methods listed. Taking the drain plugs our and filling the hose with antifreeze or get a kit from your local marine store that is basically a large tank that you fill with antifreeze and then hook this to a muffler which goes on the outdrive, water intake. Start the engine with the muffler connected to fresh water. Once the engine is warmed up (this is critical because you need to make sure the thermostat is fully open to circulate all through the motor), connect the muffler to your antifreeze tank and run the engine until you see antifreeze coming out the exhaust. I alway run it for a while after I see the antifreeze just to be sure. Even though it doesn't get that cold here in Maryland, I buy the 100 degree rated stuff assuming that there may still be some water that will get mixed with it. The first method uses less antifreeze and is probably the more thorough, however, I think the second method is fine for our climate and easier.
Air conditioning - you will see a couple of methods for this as well. I find it easier to follow the method of hooking a hose to the discharge port and use a bilge pump to pump antifreeze backwards through the system until you see it come out the intake port.
Fresh water and head are pretty straight forward if you follow the other posts.
And then I also always change the oil, oil filter, outdrive fluid, water separator & filter, gas stabilizer etc.. when winterizing
Don't know where your marina is but if you happen to be around the middle river area, I would be glad to stop by sometime and walk you through it. My marina is Bowleys in Middle River.