I haven't used this free product 'Posteriza' because I have an A2 large format printer. But if you click the link below, it will show you step by step instructions. When you reach the bottom of the instructions you'll see the link...
...highlighted in blue.
They also speak about another similar software programme called 'Posteraza', I gave that a quick look and didn't feel that looked as good. But at the end of the day it's your choice