Question
Thank you in advance for your help.

I live in CA and am in the early stages of renovating my yard to eliminate the need to water grass.  I have been told that proper preparation for installing a gravel yard, for example, requires killing off the existing grass, weeds and seeds.

Q1:  I prefer to do that without chemicals so I found two natural methods on line.  One way is to cover the lawn with cardboard and mulch with the intention of just tilling it all up when the grass is dead.  Since I have no desire to mulch/nourish/re-use the soil in the yard, I have followed the second method and covered it with black plastic.  It is also suggested that the plastic be covered with mulch and I'd like to know the reason for that.  Is the mulch just to keep the plastic in place and to keep the yard looking a little prettier, or does the mulch speed up the killing process some how?

Q2:  Also, I have been told that it takes many weeks (months) to kill off grass with this method.  How will I know that it is dead enough to move on to the next stage of lawn renovation?

Q3:  When it is dead, is it then cut (and disposed of) in order to lay down a weed prevention sheet before the gravel is spread?  

Q4:  If so, then why is there a need to kill the grass first?

Answer
If I were to install a "grass-less" yard, I would lay down an artificial (black porous plastic)cover over the whole area and follow up promptly with a 2" layer of gravel or similar inert material. This serves 2 purposes:
1. It will cover the  ground and help to prevent weeds from growing through; but allowing rainfall to percolate. Be prepared for weeds like nut-sedge (Cyperus spp. and Torpedo grass Panicum repens which will defy your every effort to control. Use Roundup?to suppress these when they emerge.
2. It will serve to keep the gravel from sinking into the ground as time passes.
Whatever you do, weed seeds will be brought in by wind or wildlife (birds especially).
Hopefully I have answered your questions.