I am finding the current state of Plutus is not suitable for my purposes, as such I wish to make a CIP.
How do I make the CIP? What should I do to optimise the CIP and maximise the chances of it being accepted?
I have not gotten our CIP accepted (yet), but I suppose following the CIP process (CIP-1), being clear on the motivation, rationale, specification and rigorous in the overall analysis, plus laying out a plan should be a good start. From there, the CIP editors and others from the community would review & request clarifications.
There is even CIP-35 currently in flight, proposed by the plutus team themselves, on how to evolve & propose changes to plutus in various dimensions!
To complete @Sebastian Nagel's answer, I would suggest to read CIP-0001 and in particular the section titled "How to champion a CIP beyond Draft / Progression to Active status".
Generally speaking, if you want a CIP to be accepted, it must obviously be well-written and contains sufficient details and justifications to be self-assessing. Yet, it must also demonstrate usefulness for the community and the best way to show this is to discuss the proposal (or the idea of it) in public forums upfront. Discussions also often happen on pull requests proposing new CIPs.
Finally, when a CIP refers to a protocol or an implementation change, it must be technically feasible, sound and secure. For Plutus, CIP-35 (tentatively) does already provides nice guidelines to drive this.