To make a spray tan finishing powder, you will need a few key ingredients that provide oil absorption, mattifying properties, and a smooth finish. Here's a simple recipe you can try:

Ingredients:
- 2 tablespoons of arrowroot powder or cornstarch
- 1 tablespoon of finely ground oat flour or rice flour
- 1 tablespoon of finely ground white clay (such as kaolin clay)
- 1 teaspoon of finely ground cocoa powder (optional, for a tinted powder)
- 5-10 drops of your favorite essential oil (optional, for fragrance)

Instructions:
1. In a clean bowl, combine the arrowroot powder or cornstarch, oat flour or rice flour, white clay, and cocoa powder (if using). Mix well to ensure even distribution of the ingredients.
2. If desired, add a few drops of your chosen essential oil and mix again to incorporate the fragrance. This step is optional and can be skipped if you prefer an unscented powder.
3. Transfer the mixture into a clean, airtight container or a powder sifter jar for easy application.
4. Your spray tan finishing powder is now ready to use!

To apply the spray tan finishing powder, lightly dust it over your skin after applying a spray tan or self-tanner. Use a fluffy brush or a powder puff to ensure even distribution. This powder will help set the tan, absorb excess oil, and provide a smooth, matte finish.

Note: It's always a good idea to perform a patch test on a small area of your skin before using any new product, especially if you have sensitive skin or allergies to any of the ingredients.