Using a Celtic Knot layout (such as the ones below), or Runes that are perhaps arranged in a way meaningful for you.
Begin filling sections with herbs, flowers, and plants sacred to the Gods or Goddesses of your choice.
You can find some lovely ideas in Patricia Telesco's Gardening with the Goddess as well as
Ellen Dugan's Garden Witchery.
Another idea widely shared across the internet is a garden with plants sacred to each feast day (known typically as "Sabbat Gardens").
There is also an elemental garden, using plants representative of each elemental direction. Scott Cunningham's Earth, Air, Fire and Water is an excellent resource, One could even go a step further and decorate the space with wee garden faery or troll statues, adding in anything indicative of the element you wish to represent.
Or my daughter's favorite a garden for the fae folk!