Basic idea is to cut up some white fish into relatively large lumps – 1.5 to 2cms, chop up some red onion fairly finely and then marinate with lime and coriander red chilli an some salt.
Probably need more lime than you might think and also less marinating time, especially if you are using fish that could be used for sashimi.
Very easy and very tasty. Probably worth making in bulk as well. Here is a guardian article on the best ceviche. Consider also adding some orange juice for a bit of a difference.
Or here is a very good sorted youtube video.