To obtain a Recruit's Insignia, you need to find and kill any Fatui at any level.
You can get these from any of the Fatui Family, such as Agents, Cicin Mages, and Skirmishers.
The fastest way to get around is to use the Waypoints, kill everything in sight, and then move on. The two images below are where you can find Fatui in both Mondstadt and Liyue.
Just click on them to enlarge them.
Thanks to Map Genie Genshin Impact Interactive Map!
- Type: Common Ascension Material
Recruit’s Insignia is used for the following Character Ascensions.
Each Common Ascension Material has three tiers; these are the next tiers after Recruit’s Insignia.