Colour your elements and start arranging them in the centre of the square. The layout should have a rhythm so areas flow together. You can layer elements, play with size, layout, colour – this is where you can get very creative and experiment.
Continue arranging elements until you reach the edges, but do not let your elements cross the seams yet.
As you get near the seams, you will need to be very careful. Remember the geometric example I showed at the beginning. An element that crosses the top seam of the square will also need to cross the bottom seam of the square in the exact same place.
The ‘mates’ must align perfectly for Illustrator to create the repeat. A good way to do this is Select the horizontal guide and the element and copy both. (Edit > Copy > Paste). Now drag the element and guide to the bottom horizontal seam and line up the guides. Use the Align dialog box to centre the elements. Now they are mates, perfectly aligned!