Try 6-10 layers of newspaper(just the regular stuff, not the shiny ads and magazines) covered by 6 inches of woodchips(you can usually find a source from a tree service, and maybe they'll even dump a whole load for free!) Best to soak down the patch of lawn well before you lay the mulch . Wet the newspaper as you apply it to keep it from blowing away. If you can wait 4-6 weeks for the grass and dandelions to compost itself, that's a plus, but I've seen successful prairie gardens planted the SAME DAY as the paper and woodchips were applied. Then push the mulch aside, cut through the paper and dig in your plants. Don't pull the mulch up right next to the stem of your plant, but get it close. Keep well watered till plants get established.
Diane Dodge in St. Paul, MN