These posters are made with A3 sizing, the way I did it was using Adobe Acrobat Reader DC, I printed them on two A4 pages, so they printed one half on each page. If you press print – multiple, you can adjust the sizing of the page, so it prints on two A4 pages.
Then I trimmed the edges and stuck the sheets together. I have an A3 laminator, so I laminated the poster using an A3 laminating sheet and matched the two pages, so they printed perfectly together. You do not have to have an A3 laminator you can easily print and laminate 2 separate A4 sheets and sellotape the back of the 2 pages together.