Click on any Sim (child or older) in the world and look for the Fashion Authority category, select Create Thread.

Body Frame and Fashion Preference will be pre-selected based on the Sim you targeted.

Select Generic, Situation Job, or a Career restriction for the uniform. Situation Job will show any of the jobs the Sim is currently assigned to. Careers will show any careers the Sim has joined. Generic uniforms will not have a job/career restriction and are not automatically assigned on spawn.

Next, you can choose whether to Start Fresh which will set a plain outfit on the mannequin, or you can copy the Sim’s current outfits, or the entire category onto the mannequin.

The Edit Thread menu will then appear where you can make additional changes and Modify in CAS at any time. I recommend you first Rename so you don’t lose track of your new Thread after leaving CAS.