This is an annual favorite 1st grade project. I approach it exactly the same way each year, because it works so well. The only change I make year-to-year is how to paint them.
It's so easy; I hack up cardboard shipping boxes, and the kids use Elmer's Glue-All that they paint on with a pop stick. They need to count to 10 as they hold, each time they add a new piece. We spent 2 sessions building and 1 session painting. Some students (such as the bottom 2 photos) will need a second painting session to complete their sculpture.