How do I get a victorious skin?

It's for Sivir this year.

You have to get to at least Gold, without boosting and without getting any restrictions on your account.

Yup. Gold + players get the skin unlocked. I also think you need to have the champ unlocked too. Although I am uncertain...

Get to gold or higher. It is Sivir. No long bans and you have to have no chat restrictions currently active

Get Gold in soloQ or participate in 10 wins on a ranked team that is gold or higher (both 3s and 5s).

Get gold in any ranked queue.

Must not be under ban or punishment when they are given out.

The reward will be taken away if you shared your account (aka got boosted).

The skin is for Sivir and she will be unlocked if you don't own her and you get the skin.