Possessives
Possessives use endings like ein: mein Vater, meine Mutter, mein Kind. In accusative masculine use meinen; in dative masculine/neuter use meinem and feminine meiner.
Understand the idea
A possessive has two jobs. Its stem identifies the owner, while its ending matches the thing possessed. Mein refers to me as owner, but the ending changes according to the gender, number and case of the following noun.
Build it step by step
Choose mein, dein, sein, ihr, unser, euer or Ihr for the owner. Then apply the ein-type endings to the noun phrase. Masculine accusative needs -en, dative masculine/neuter needs -em, and dative feminine needs -er. In plural, possessives take endings even where the singular ein has no ending. Euer normally loses the second e before an ending: eure.
See it in a sentence
Ich sehe meinen Vater.
Das ist meine Mutter.
A useful memory aid
Ask two questions in order: whose is it, and what form does the possessed noun require? Keep those decisions separate.
A common trap
Do not choose sein or ihr from the gender of the object. A woman's masculine noun still uses an ihr stem; a man's feminine noun uses a sein stem.