I'm about to do this against my local police force for assault; I've taken advice from my insurers as I have legal expenses insurance. In case it might end up with you suing the police, check your house contents insurance to see whether you have legal expenses insurance, it's often with a different, more specialist insurer, than the one insuring your actual contents.
You have to complain to the offending police force as a first step, and the complaint will be investigated by an officer one rank higher than the one(s) you're complaining about. If not satisfied you can then appeal against the outcome, when the complaint will be reinvestigated by the next rank up.
Only after this, if you're still dissatisfied, can you appeal to the IPCC. If you do complain to the IPCC initially all they will do is send your complaint on to the force anyway, after recording that a complaint has been made. The police have to notify the IPCC when they receive your complaint, that they've received a complaint and the date, but nothing more.
Put everything in writing, tell the police specifically that there will be no phone calls, no personal interviews, no personal contact at all. This is because the police are very bad at investigating their own, and lower level officers especially may well reveal further evidence against themselves. It also stops them arguing 'We didn't say that', 'You didn't say that', 'It didn't happen like that'. You need a paper trail of every detail. My insurers were really emphatic about this. There's also some decent advice online about how to word a complaint; being strictly factual, unemotional, and asking the police to take whatever action they think appropriate rather than suggesting a punishment are very important.
Download a copy of the Police Code of Ethics. This is a statutory document, the police are legally obliged to obey this Code and it helps to be able to identify which section of the Code they've breached. It's not a long document considering what it is, 15 pages including the cover and contents.
If your claim is worth suing for (mine is, I've had Counsel's advice on that) then your legal expenses insurance will kick in when you've exhausted the first two levels of complaint; you have to do the initial complaint and appeal yourself but having a decision go against you at those two levels in no way stops you being able to sue if you feel it's worth it.
The incident I'm going to complain about happened a year ago but I have it all documented and will start the complaint when I've settled my personal life, I'm moving house in the week after Easter and it's been hard work, with a difficult seller to deal with. I have three years to start legal action from the date of the assault.
HTH.