Going to try and get into another race soon hopefully, can't really sign up to do full series racing so just pick and choose what I can do.
Great thing about cross - seems to be very easy to get into and very inclusive, and you can just pick and choose events to enter and sign up on the line, don't even need to be a member of a club or BC, as mentioned upthread (although I am both). I'm doing the one this weekend then I'm hoping to do two more this season. Maybe next year I'll get into it more seriously.
To go back to the OP re practice: the only practice I've done is riding round trails in the woods, and at the park where they have the local CX races. Nothing quite prepares you for the adrenaline of actually racing but it's good to get a bit of a feel for the terrain and handling if you're only used to road riding (I've never been into MTB either, though I did a bit of BMX in my youth).
In my very limited experience, what
@screenman says is very true:
The nice thing about cross is there are many races going on, there will be somebody there of your level that is for sure, do not put of starting the season is too short. Nobody will take the P if you come last, they will though admire you for having a go.
Last race I did, I finished comfortably mid-field. Got lapped twice by the top few riders but they don't care about that and won't judge you for it, as long as you don't get in their way when they're trying to get past. I also lapped a few riders myself.