The message
from "Joe98" contains these words:

Hmm... What you suggest sounds like a good tip for making good loam, but
is that necessary? As I said, I have grass on the area designated for the
garden but it's spotty and useless as sod. In the process of the Double
Dig, I was planning to just work the grass into the mix.

Yes do, but be sure its at the bottom so it won't regrow; see below

After I Double
Dig, I was planning to cover with plastic and leave it for all of winter.
Is there anything wrong with doing it this way? It would save some work,
which is important with old man winter coming around the corner.

I'm just in the middle of doubledigging a new bed in what was lawn,
like this;

Skim off three rows of turf/sod/grass by cutting it into tiles an inch
or two deep with the spade, then sliding spade underneath. Put them
aside on a plastic sheet, out of the way of your working area. Dig out
the uncovered soil two spits deep and put it aside, not on top of the
turves you just lifted. Now skim off your next three strips of turves,
and as you work put each one upside down in the bottom of the hole you
dug. Spread on the turves whatever soil improver you're using; don't
worry much about proportions but try to spread each material evenly.I'm
using seaweed, rotted horse manure, and a LOT of coarse grit because our
rainfall is high and this is to become a freedraining bed for sunlovers.
Dig 2 spits of soil you exposed when you skimmed off the turves, and
roughly pile it into the hole on top of the upsidedown turves (it will
look mountainously high, but will settle later).Keep repeating this
pattern.

When you come to the end, you will end up with a hole where you just
finished digging. Fill it with the first turves and first soil which you
put aside.

It's hard work, so to protect your back and joints stretch and warm up
beforehand and take it slowly, with plenty of rests. Make an effort to
change which foot you're digging with, on each row,to even out the
muscular strain.

Covering the finished bed with (black)plastic will stop weeds
germinating during late autumn; but I prefer to let them germinate so I
can easily hoe them off before planting the bed. Also, you really want
the winter rain and frost to break up all the lumpy soil and settle the
bed down.

HTH

Janet.

Isle of Arran, Scotland.