If a dealer will do it for under $100, then go that way.

Otherwise, be advised that the block drains are aft of the motor mounts, and screw straight up into either side of the block. Be advised they often require single-use washers of a particular material.

Beyond draining the block via these drains, I'd also undo the hoses going to/from the heater core, and insert some sort of extractor hose to suck any coolant out of there. Also, unhook the lower radiator hose, as it's lower than the drain.

Your system is self-bleeding; in my experience, I added equal parts coolant & water (via a disconnected upper radiator hose)until the system stopped looking for more. Hook the hose back up & fire up the car; keep the radiator cap off at first in case you need to add additional fluid. When warm, turn on the defroster full blast (once car's warm) and that'll get fresh coolant into the heater core. Turn the car off & let it sit; in a few minutes it'll burp and the coolant level in your expansion tank will drop. Top off & you're done!

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo