I converted my entire front yard which had an established lawn, and planted native Texas plants. it is a beautiful garden now.
I used 8 sheets of newspaper instead of cardboard, and put chipped tree mulch on top, which I got from the local tree trimmers for free.
Cardboard will work just as well, but be sure to wet it after it is down, and do a small area at a time, so it doesn't slide around. It is crucial that you have a good overlap on the pieces of cardboard, if you don't the light will reach the grass and it will grow between the seams. Also if you try to tackle too large of an area before you put the mulch down it will probably get blown and slide around.
Mine took about three months to completely die, but I never had any problem with the grass coming back.
After the grass was dead, I planted the plants by pulling the mulch aside and making holes through the newspaper and then putting the mulch back.
Don't rush your planting, or you will be fighting that grass for a long time.
Good luck with your project, it is very worthwhile.
