Combine sugar and water in a small saucepan set over medium-low heat.
Swirl the pan, encouraging the sugar to dissolve. Once sugar is completely dissolved, remove from heat.
Meanwhile, crush juniper berries, fennel, and cardamom with a mortar and pestle. If you don't have one, no worries! Place ingredients on a cutting board and crush with a flat surface (a coffee mug, measuring cup, or saucepan would work great!). You're really just wanting to crack the seeds open, which will allow them to infuse the syrup.
Add crushed seeds and pods to the syrup. Allow to steep for no less than 30 minutes: note that the longer it steeps, the stronger the flavor will be!
Once your desired flavor is reached, strain with a fine mesh strainer in to a glass jar and store in the refrigerator for up to 1 week.