First you need the 'add charm' skill, you can find it under the warior skills. For each point you invest in this skill, u can add one charm. ( when your 'add charm' skill is level 3, then u can on highest put 3 charms in an item.)
Second you need stuff which has place for putting charms in them. Some items have place for 2 charms, while others have place for five. Only in those stuff you can add charms.
For example, if you find a belt where u can put 3 charms in. This would mean you can maximum put in 3 charms. If ur add charm skill is only level 2, that means you can put only 2 charms in the belt. If ur skill is still level 2, and u find a pair of shoes with only 1 place to put a charm in, then you can only put one charm in it.