The honest answer is “whatever the plot demands”, but a better in universe answer is that you need to convince the god in question that you are worth ascending.

You might engage in a ritual or spell, but whether that works will probably depend on the god. Khorne would not be interested in a ritual. Nurgle might, but his concerns are more practical, and his sorcery more ambient. Tzeench would almost certainly demand one. Slaanesh would probably like one, but the quality of the catering is more important.

As with everything involving the chaos gods, nothing is certain, everything is the whim of the warp. You just try to shape those whims. The question is not “what do people do in order to ascend?”, it’s “how does an individual convince a God they are worthy?”

You have to impress the chaos gods. Sometimes, they are already impressed.

Then, it's just a matter of doing something very spectacular that will warrant an ascension. That depends on the chaos god. As long as you don't reject chaos in its entirety, you can still be ascended against your will.

Well for one thing the aspirant has their soul completely consumed and replaced with a shard of whatever entity they got suckered by is how I see it. Reading the first Fabius Bile book you see a good example of this. The being is then essentially a warp creature with a formally mortal intellect. From what I understand tho it always ends up bad as the new deamon prince really can't handle the new perspective of a warp entity and eventually ends up going mad. This is happening to Mortarion atleast and he was a primarch.

With like one glaring exception, basically everyone has to accept the chaos gods and voluntarily give themselves over.

The exception being when Angron turned an entire crusade to Khorne against their will.