Two-dimensional construction work is all about assembling materials on a flat surface to create a design or image. The work can be very free e.g. tearing up pieces of coloured paper and assembling them in a free pattern on a sheet of paper (collage with infants) or very directed e.g. using specific shapes of card/paper to produce an image (mosaic work with senior classes).

This is, generally speaking, the freest form of two-dimensional construction.
Papers/card/other materials are torn or cut into random shapes and assembled on a backing sheet.
The types of activities involved might include

Montage is completed images used to create something new.
Images can be cut from magazines, papers, old books, photos, postcards, etc. and stored in categories for use in this type of work. Examples of such work might include

Mosaic is a more directed activity and involves using small tiles of paper (usually in squares) to create a design or an image. It is probably best suited to senior classes. Children can see examples of mosaic work in Roman floors, Islamic art and in churches. Examples of work might include

Follow up.

Work on a group collage with your friends where you will use a range of materials including papers, fabrics, paints, etc to create an image based on a famous story.
Use drawing/colouring materials to produce a range of designs for a tiled floor.