Drawing and careful planning are critical to developing a repeating pattern that works.

There are a number of ways to make geometric layouts. I often use quadrille paper for basic sketching and tracing paper for layout and "mirroring" objects that are repeted or mirrored along either axis.

This is an example of a simple sine curve run across the 1/3 point of either end of a 4" by 6" tile pattern (bottom). Shown are two different floral patterns developed from this basic curve.

Draw the initial part of the pattern on tracing paper staying straight and one one side of your axis (division).
Fold the tracing paper over with the fold exactly o the axis.
Trace the pattern and open up the paper. This works like cutting paper dolls. You can use horizontal and/or veritcal axes, or diagonal, or multiple. Play with it.

A computer and a drawing program can also be handy, but is in no way necessary. I used one on this, not for the initial design, but only to "clean up" my lines and to make it easy to print multiple copies.

You will use paper copies to trace your pattern onto the wet clay tiles.