Link to PDF
Some other resources:
Photoshop Treshold Effect
Take your ORIGINAL collage image and in Photoshop
1) Change to Grey scale (IMAGE>Gray Scale)
2) Go To Image >Adjustment > Threshold>
3) Adjust the slider to value that looks good with your design
Any Gray areas in design could be converted to halftone via bitmap.
Select one of your motifs in half tone and print /copy on two transparencies A4 size.