Start by drawing templates for the future Christmas tree, or you can draw on green cardboard. Cut out the blanks. This is how it turned out for me: herringbone template with mandatory indents - edges for gluing on cardboard
And there should be four Christmas tree sides
And in order to cover the bottom, there should be a square blank. Glue all the templates to the square on the edges
And assembling our Christmas tree
Glue the branches together, trim any excess bumps. It should look like this
Well, and if it will be necessary to make the Christmas tree hanging, we make a loop from tinsel
You can decorate this cardboard Christmas tree as you see fit.