Making a layout

Depending on the amount of photos you have, you need different layouts.
For using just one photo, it is best to centre it.

To make the layout, open a new canvas (if you are making this from the canvas, make the new canvas 700mm x700mm)
Drag the photo across to the new canvas.
View > Show > Grid. Make sure it is on Snap to Grid.
Make the photo even on both sides and take the grid off.