Start by cutting out the inner shapes.
Don't worry if you don't cut exactly on the lines: this is the back of the sheet, which will be turned once you are done.
Be sure to use a properly sharpened blade and keep the paper still while you are cutting out the shapes to avoid tearing any of the finer details.
Cleanly cut out each edge of your design using a ruler and craft knife.
Apply glue to the back of each sheet and stick them together, matching up the edges.
Use a paper glue that is low in water content to avoid wrinkling the paper (glue stick, gel or aerosol spray).
Glue your scene (consisting of three squares) into the middle of the pea green A4 sheet.
Then, cut away any excess material to obtain an even frame around your picture.
Depending on your printer, this will give a border of approximately 1.5cm.
You could either frame your collage or send it as a card... See what ideas you can come up with!