Download the Rasterbator software from the download link in the Resources box at the bottom of this article.
Double-click on the installer file you downloaded, and click "Next" through the prompts until it is installed. Click "Finish" at the end, and the installer will close and open up the Rasterbator software automatically.
Click the "import" button, and select the image you want to make into a poster.
Select the number of pages you want to use to print the poster.
Check your printer ink levels to make sure you have plenty of ink left.
Click the "Print" button.