Just go to the Riot Suppot Page -> Submit a Request -> Limited Free Limited Time Promotional Skins: Tristana/Alistar/Garen.
There you can select up to three skins which will be added to your account after submitting the request.
Note that this applies only till the first of August so you better rush. After this date the Skins will be vaulted.
All the Upvotes and credits go to the creator of this post:
Have a nice day.
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