If you get S rank for a champ for the first time in the season, you will receive a chest, which you can unlock via collecting key shards overtime and combining 3 shards into 1 key.
Chests will potentially give champ skin shards, which you cam make it permanent via Orange Essence that may either come with the check unlock in small amounts, or you will have to disenchant some skin shards.
If you do not have enough Orange Essence to unlock the said skins, which everybody is short atm due to the infamous essence nerf, you can grab 3 skin shards and "reroll" into a random pentameter skin.
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There is no way to obtain skins via Blue Essence outside of like URFwick which would be available in BE shop (the one that opened up during rune refund) if they make it annually like they said they may do, which the skin costs like 1 million BE or something insane. Though BE can be used to purchase chroma in the BE shop too, so can be used as substitute for skins if the chroma is available for a champ's Classic skin.
Otherwise free skins can only be collected via giveaway codes that random people may do on stream or whatever, going Gold5+ for the Victorious skins at the end of a season, or things like event missions that by completing the missions may grant players free icons, ward skins, or chromas through the completion of quests, and potential skin shards via token exchanged event specific chests to open.