1/4 cup lemongrass, pale green parts only, roughly chopped (about 2 stalks)
1 teaspoon coriander seeds
10 mint leaves
1 allspice berry
1/2 teaspoon gentian root (cut, not powdered)
1/4 cup celery seed
1 cup celery, chopped
2 cups 100-proof vodka

Put the lemongrass, coriander, mint, allspice, and gentian root in a sealable glass jar. Muddle briefly to bruise and break the ingredients. Add the celery seed, celery, and vodka, then seal and shake. Let this mixture steep for 12 days, shaking daily.

Strain solids. Add 1/2 cup water and cook on medium for 5-7 minutes until it reduces slightly. Steep 2 more days. Strain.

 

Source: https://www.seriouseats.com/recipes/2012/04/homemade-celery-bitters-recipe-cocktail-ingredient.html