If you are doing this because you need one go buy one. It will lekely
cost you less that the parts to build your own.
If you are making this to teach yourself how to do it then:
(1) Decide if you can live with a near square wave or need a sine wave
(2) Decide if you need to handle very bad power factors
(3) Decide if you can stand having the 220V vary with the battery
voltage or not.
(4) Decide if you care about how big it is.
If you take the easy way on all of these items, the design is very
simple.
To add to this post:
Many times I build gadgets because it is fun and I like doing it. If all
you can do is BUY everything, you are not really living. For me it is
satisfying to actually build some of the stuff I use. And I get most of
my parts from the town's recycling center. Yeah, I depend upon the BUYERS
of everything who toss what seems not to satisfy them for the moment.
And if it doesn't work, hey, that's life. Press on to the next project.
So don't knock people who want to build something. Give them assistance
and credit for their industry.
Al