Traditionally you layer fire safe rocks on the bottom, corn on the cob in the husk, veggies (new potatoes, cabbage, carrots, celery, onions, etc), then meat (often sausage such as kielbasa). Beer or possibly broth is poured over all, it is capped with a VENTED cap and placed in the fire for about an hour. When done it is poured out onto a newspaper covered table and everyone dibs away at it.

Updated, you use a steamer rack or handfuls of canning rings in the bottom of the cream can to keep the food above the beer.

This site gives the straight up info on a 10 gallon cream can dinner, which feeds 30+:

barbequelovers.com/grills/milk-can-dinners/cream-can-dinner-recipe

This is a VERY good instruction, use it for your basic info, even with the roaster.

To adapt for a roaster, this is what I would do:

1) An 18-20 quart roaster is the equivalent of a 4 1/2 gallon can, so plan on at most 1/2 the amounts, feeding about 16.

2) Nesco roasters have the heating element in the sides, which is NOT the best for this recipe. One with the element on the bottom, such as the Hamilton Beach, is what you want.

2) Put a heavy rack in the bottom of the roaster that covers the whole bottom. You want the liquid to sit under the food, not the food sitting in the liquid. You can measure how much liquid to add by setting this up. putting in water up to the rack, then measuring the water. You will replace the water with your steaming liquid (beer or broth) when you do the recipe.

3) If you have any doubts about the depth of the rack, put the corn in the bottom layer to lift up the other food.

4) Layer in the food as discussed in the recipe, meats on top. One potato where you can poke it to check doneness. Then the future steaming liquid.

5) Put the lid on, and then leave it on until the cooking is done and it has sat the required 5 plus minutes after.

6) Turn on to 325-350.

7) Start timing from when steam comes out the vent hole. 30 minutes. Turn off, let sit 5+ minutes.