For the formation of the past participle, we conjugate the verb separately from its prefix. Then, we attach the prefix to the beginning of the past participle.
Afwerken (to finish, to give the finishing touch)
prefix + past participle = af+gewerkt = afgewerkt
Goedkeuren (to approve)
- ge+stam+d = ge+keur+d = gekeurd
- goed+gekeurd = goedgekeurd
The list of separable compound verbs is very extensive. You can have a look at the most common separable verbs in List of separable verbs.