I've taught myself to do a number of activities left handed including hammering, sawing, shovelling, raking, pushing broom and swinging picks/axes/sledges. It's a good idea just for the benefit of building both sides of your body symmetrically. You'll have less back issues, too.
When I'm learning a new activity I will study closely the way I've been doing it right handed. Body position, arm position, hip angle, etc..... It all goes into the work.
For learning to hammer there's nothing like a few boxes of nails. Start with some 8 penny nails. Drive a couple dozen every day for a few days. When you're starting to get comfortable with those then go to 16 penny nails. The 16 pennies will require harder blows and you'll start to gain power in your off-handed blow. When you're comfortable hitting nails then start swinging at things on your anvil.