There isn’t really a way to do this right or wrong. There are standards…
Well to the East
Fire to the South
Tree to the West
But apart from that, ADF Druids (and the like) usually decorate their alters in ways that makes them feel connected, happy and spiritual.
You can use seasonal herbs and flora or candles and crafts. Most people incorporate symbols or effigies, but I will tell you my family alter is often quite simple and apart from the well, fire and tree I usually end up decorating it with things I’ve made. I mean, you always want beautiful flowers around Ostara but here in Nova Scotia you would be hard pressed to find the ground though all the snow let along a flower! I tend to make paper flowers ( I love papercraft and I build with intent), we have homemade candles and herbs that we often grew in our yard at some point.
Your alter need not be complex or garish unless you wish it so. Use what represents you and your deities.
If you are looking for inspiration, here are a few ideas:
Statuary
Precious books
Crystals
Ogham
Runes
Your well, fire and tree representations
Incense and holder
Candles
A tablecloth
Herbs
Flowers
Magical tools of any kind
Your chalice
Your offering bowl, plain or otherwise
Homemade (or lovingly made) crafts
Seasonally appropriate colours or figures (eg: reds and browns for Mabon etc.)
This is a very personal process and you should always do what resonates with you and your needs at the time. Be sure to cleanse whatever you place on your alter and always focus your intent as best you can.
Blessed be and happy decorating.
Karen