How do you create a grass plot into a vegetable garden?

You can lay cardboard over the area you want to place your garden, then cover the cardboard with a layer of topsoil, then a layer of mulch. Let this sit for at least a month, watering if it doesn't rain much. You can then push back the mulch in the spots you plan to plant. I don't recommend using roundup to kill the grass as it will remain in your soil and impact anything planted afterwards.

I am in the process of doing that right now. Instead of digging down and trying to get rid of grass, my son got lime and covered the yard to change the Ph. Next step was to cover the desired plot with black plastic bags (tarps) to kill the grass. We then built the raised garden beds ( used Lincoln log method) And, finally had loads of fresh soil into the raised beds. ( 6 loads) Tada. My only problem was the passerby's who wanted to offer advice and to learn where I was planting what. But, I have already started planting spiky edible fruit bushes to act as a natural fence. The Piece de resistance was planting Carolina poppers and ghost peppers along the exterior edge of my beds as well as plants that are non edible (witch hazel) or toxic until prepared.