So how to create the Past Participle for verbs in the present perfect tense?

Anytime we have to say the following sentene in Spanish we need to convert the infinitve verb to its past participle form:

I have eaten here before

You have read my other Present perfect tense/past particple posting so you know that how to conjugate 'have' but what about 'eaten'?

Well, it boils down to this: Regular vs irregular verbs

Regular verbs follow the next rules:
1. remove the verb ending (ar, er or ir) and replace it with -ado or -ido: -ar ending verbs use ado and -er or -ir ending verbs use -ido

2. Example: Cantar converts to cantado, comer to comido and recibir to recibido

So if you want so say 'I have eaten here before' it would translate to 'yo he comido aqui antes'

Irregular verbs:
sorry, most irregular verbs have no specific conjugational process, you just have to learn them, for instance
hacer   hecho
abrir    abierto
escribir  escrito

I hope this helps