You already know my, your, her. When you do not want to say the thing again, swap to mine, yours, hers: my book → mine.
my book = mine
This is my book. = This book is mine.
That is your seat. = That seat is yours.
It is her bag. = The bag is hers.
Not "This book is my." — say "This book is mine.". With no thing after it, my becomes mine.
Try it: That seat is ___. (yours / your / your's) Answer: yours — Nothing comes after it here, so use yours, not your (your needs a thing after it, like "your seat"). And there is no apostrophe: yours.
The whole set
Here is the matching set: I → mine, you → yours, he → his, she → hers, we → ours, they → theirs. Note: his stays his, and there is no -s on mine (not "mines").
Is this pen yours? No, it is his.
These tickets are ours, not theirs.
The red coat is hers.
Try it: This phone isn't yours, it's ___. (mine / mines / my) Answer: mine — The word is mine, with no -s on the end (not "mines"). And here nothing follows it, so not "my" either: it's mine.
Nothing comes after it
A word like mine, yours, hers stands alone. Do not put a thing after it. If you want to name the thing, go back to my, your, her.
Those keys are theirs. (not "theirs keys")
Is this yours? (not "yours bag")
It is her phone. = The phone is hers.
Not "That is hers bag." — say "That is her bag. / That bag is hers.". Name the thing → her bag. No thing after → hers.
Try it: Those are ___ keys. (their / theirs / theirs') Answer: their — There is a thing right after it (keys), so use their + keys. "Theirs" stands alone with nothing after it: "Those keys are theirs."
Try it: Is this ___ umbrella? No, I think it's ___. (your … hers / yours … hers / your … her) Answer: your … hers — The first blank has a thing after it (umbrella) → your umbrella. The second stands alone → hers. A thing after it takes your/her; alone takes yours/hers.