i've bought some roadie overshoes and from what I can see it looks like I have to cut out the hole for the cleats? there are already some slits on the sole but I guess you cut these to suit?

overshoes usually have the holes in them already, if you cut the finished edges they will be trashed in no time!

They won't be trashed in no time… 😕 Put said overshoe on, position it as required, use a sharp blade to cut through and reveal the cleat. About 1" should do.