It's harder than you'd think it'd be.

Slaanesh is about doing something past the point of all sanity.

Let's say you like eating chips.

Now you could eat tonnes and tonnes of chips, but the chances of catching Slaanesh's attention from that is very slim; as everyone can, and probably is, doing that.

No, you would have to spend months perfecting the cooking method and getting the right balance of salt and vinegar.

You would have to create the chips with the crunchiest outside and the fluffiest insides served up with the richest tomato sauce and the finest salt and vinegar.

You would then have to eat one of these most perfect of perfect chips….and then throw the whole lot away because they weren't good enough.

That is the sort of lunacy you would need to do in order to earn Slaanesh's favour.

In all honesty, the easiest one is probably Khorne.

Kill shit, collect skulls and don't be a cowardly and dishonourable bitch.

Tzeentch will fuck with you for shits and giggles and you'll end up a semi-amorphous mess at the end of it.

Nurgle is a bit of hit and miss, as once you've earned his favour, you won't feel the effects of his plagues and diseases; but you will need to endure and survive up until that point, which is the tricky bit.

There is another option available though. One the Dark Gods would rather you not know.

You could be a complete utter Madlad and go with Malal.

Just running around the Chaos Wastes fighting Chaos in the name of Chaos and hoping that the Chaos God of Fuck You can break through the veil of Dubious Canonicity to grant you insane levels of power.

As Malal has very, very few followers, he can afford to power up his champions to ludicrous degrees and because he has so few, it's much, much easier to grab his attention compared to the other gods.

You had just better be prepared to serve the Chaos God of Anti-Chaos and all of the contradictions that implies.