Step 9
Do the same thing for the left and right side seams, making sure everything line up exactly. Select the left vertical guide and the element, copy, paste, and align as described in Step 8. Do this for all elements that cross the vertical seams.

Make sure the layering is exactly the same along the left and right seams. It can also be helpful to Lock (Object > Lock > Selection) each item as you align them so they do not accidentally get moved. Even if even just one element gets nudged it’s going to haunt you in Step 13!

Step 10
Now that you have copied and pasted your elements so the mates along the seams match you are almost ready to create the swatch. Just keep in mind that once you’ve created the repeat swatch you can only alter its scale, not the colours or composition. Now is the time to make any layout adjustments and colour changes. Select all elements and group them.

Step 11
Make a copy of the white background 30 cm square. This should have no fill and no stroke. This will define where the seams of the pattern are when Illustrator generates the repeat swatch. Send this invisible square to the back using Object > Arrange > Send to Back. Use the Align palette to make sure it is perfectly beneath with the white background square in the pattern.