First, obviously, lay out your garden plan. I use a garden hose or rope to rough in the shape. I almost never do gardens with straight lines.
Another truly clever thing to do is let the lawn grow up just a bit, and then use the lawn mower to mark off your curves. That way you are assured a curve around which the mower can move easily, and avoid that mower dance of back and forth, up and down, lift, swivel.. well, you get the idea.
What about all that grass? Well, you'll want to get rid of it. Every last rhizome you can find. All of it. You can lay plastic down for a few weeks and thus kill off the grass and any sneaky weeds, or do the same several layers of damp newspaper and a few well placed rocks. If you don't like the look of this, you can throw a few shovels full of dirt atop it, and disguise the process.
Otherwise, you'll have to rent a sod cutter or use your spading fork and do it the old fashioned way. If you are lazy, you can turn the sod upside down as you work, cover it with plastic or newspaper and leave the actual digging of your garden until next early spring.
If you want to get your bed going as soon as possible, then remove the sod, knocking off some of that lovely dirt back into your garden, and compost the grass. This step is probably the most tiresome one in terms of monotony and hard work (at least it is to me) but if you get the grass out now, you won't be fighting it down the road.
So now what? Well, you will want to work your soil. The old timers called it tilling. I guess we still do, so let's pause for a moment and salute the old timers. Can you rent a tiller? Absolutely. The only thing to avoid is tilling yearly. In fact, if you are using a motorized tiller, you should till a garden deeply once, and then leave it alone if you've been good and haven't walked on it. Over tilling can actually create a layer of hardened soil down just beyond where the tiller blades reach, so unless you are going to be putting in tons of gardens, just rent a tiller and use the money on soil amendments.
