How to form the passe compose with avoir

Let's use the word manger (to eat) as an example, the sentence will be I have eaten

First: let's get the pronoun 'I':

je 

Next, we take the auxillary verb (avoir- have)

ai

then we take the infintive of the verb :manger, remove the e and replace it with an é and we have the verb :

mangé

now we join the 3 words to make a sentence

je ai mangé

but in french, two vowels (e and a) cannot be together so we use an apstrophe in the place of the vowel which looks like this

j'ai mangé