It's quite complicated to do with full cardboard. But... you can get it done a lot easier by removing the top layer of paper from cardboard and shaping that to dome shapes or other geometric shapes. I show you how step by step.

We make a cardboard dome in this tutorial. I show you how to do it. And the one we make is for my World's Greatest Cardboard Castle project

I have a smaller building attached to the front of a larger building. Here is a drawing of it.

This is what we are going to do. Add a small rectangular structure. And at the top of that structure we will add a partial dome.

We will remove the top layer of kraft paper from a piece of cardboard and shape that to our dome.

Start by carving a piece of styrofoam or foam to the shape you want. This will act as a mold.

Now let's get a nice sheet of kraft paper by removing it from cardboard. Apply a liberal amount of water to the top sheet of paper. Make it nice and wet. Cardboard is assembled together not with glue but with starch. And that starch dissolves in water.

Let it soak in for three to five minutes then try to peel that top layer off. If you have trouble just go ahead and brush on more water. And as you are peeling it you can add more water to any tricky spots.

It looks good! Don't let it dry. Let's move on to the shaping of the dome.

Mix up some white glue and water. Make it about a 50/50 mix.

Apply that glue/water mix liberally to the sheet of kraft paper.

Shape the kraft paper over your styrofoam mold and use toothpicks or push pins to hold it in place. There are going to be wrinkles. You can make slits in the paper to minimize wrinkling and to help achieve the desired dome shape.

Apply another liberal coat of water glue.

Once you have it to the shape you want let it dry. You can accelerate the drying time with a fan, hair dryer or heat gun.

Once it is dry you can remove it from the styrofoam mold and trim it. You can also do further detail work to it with tape and scissors. And you can wet it in spots with the glue/water to soften it and work it more.

And here you go. I have glued it into place.