Fill a saucepan with a few inches of water and place on medium heat (or use a double boiler).
Add the first five ingredients (all except the zinc oxide and essential oil) to a double boiler (I use a quart-sized glass jar).
Put the jar with the ingredients in the pan of heating water.
As the ingredients start to melt, stir to combine.
Remove from heat and allow mixture to cool a bit.
Add in the zinc oxide and essential oil to the jar or double boiler (I use 10 drops of lavender and 5 drops frankincense, but you can try other combinations. The only caution is with citrus oils, as they can increase sun sensitivity.)
Whisk the ingredients well with a fork. As the mixture continues to cool, stir a few more times to make sure it is well blended.
Pour the finished sunscreen into a glass jar for storage, and store at room temperature.
Notes
To use, apply a thick layer to skin before sun exposure. Be sure to reapply sunscreen every couple of hours, more often if you're in the water.