Its easy when you know how... in this video I demonstrate how I define a pattern and then check that it is repeating seamlessly.
Before I started I created a 20cm square file and arranged all the elements on the tile.
I use "duplicate" and "transform" to make sure all the elements are in the right spot before I define my pattern. I'm planning to make video to demonstrate this too. If you prefer to learn in person, then please consider coming to my workshop "Digital Design for Textiles".