Thanks for the reminder about the cardboard layer.

But I thought the unfinished compost and loose leaves and grass clippings should then go top of that cardboard and then completely covered by topsoil which would then be more or less composted by next April?

I always overlap the cardboard by at least 4-6" to make sure the grass can't find a crack through. I would put the leaves/grass as the top layer. This most closely replicates the natural levels of soil, the way mother nature does it by dropping leaves, which then decompose slowly and are then covered by more leaves.

The soil on top of the cardboard really isn't necessary but if you want to build a border to contain all of this added material, it should be a very nice raised bed. I fear it will erode away to the sides without some type of border. Adding soil will also hold the cardboard firmly to the contour of the ground underneath, most effectively smothering the grass. Not a bad idea at all.

Buying soil is probably not the best thing gardeners do except for those that sell that soil.

How do you expect me to do this new bed?

One of the composters you see in my picture is full to the top with much of the contents close to fully composted. I also have several bags of mulched leaves and grass. All of this aforementioned material will be dumped and spread on the lawn in the indicated area of the new garden. If I can easily get some cardboard in the next few days I will have that as the first layer.

But given I want to have the planting part ready in time for next April/May I am starting now and I have no choice but to supplement the compost, grass and leaves with purchased earth. Although the indicated area is not large (I havent decided on the exact edge but it will like encompass about 130 sq. feet). The drop off to the back coupled with the more or less raised nature of the bed will require probably 2 cubic yards of soil or so which I will need to buy.

You can certainly put layers of compostables under or between soil layers, in case that was in question. Yes they will break down more slowly, but the decomposition process itself provides a rich growing environment. Just make sure whatever you put in is roughly C:N balanced.

If you have mostly dry stuff ("browns") sprinkle in some high-nitrogen fertilizer as you make the layers. End by watering it well to make sure everything is wet.

In the spring, build the rest of the wall, tossing the excavated dirt into the middle of the bed, then level the bed out and add dirt if needed.

This is not like making pastry - proportions are not critical, especially when you have all winter to let stuff sit and rot under the snow. There are bacteria and fungi that work best at low temps - not as fast as the thermophiles, but they can still get things done.

Both bins are where they are as it was the best place for them at the time of setup. After much soul searching ;) I think I can relocate one of the composters but if not the other than I plan to have a very informal stepping stone pathway to access the remaining one. Of course I need to be sure to look after this piece of the puzzle prior to putting in plants.