Unfortunately I didn't have any warning at all, no spotting or cramps.
The first time I was walking home from shops, felt a rush (thought I'd wet myself, or maybe waters) and only when it came through leggings I realised it was blood. Second time I just got up from bed and felt the rush, but knew what it was, and third time realised as I turned over in bed. Completely painless and felt fine (although shocked) each time.
I've been told to phone the ambulance immediately, to say its placenta praevia, that it's urgent I get to the hospital and I may need an EMCS. The ambulance will phone the birthing centre.
I live about 35mins normal drive from the hospital and each time it's taken about an hour from calling the ambulance to arriving at the hospital (inc paramedics doing obs). The 2nd time was a large bleed (475ml) but I was still fine on arrival. That time I was given a drip, steroid injection and magnesium sulphate, just in case, but the bleeding subsided and baby was happy so no need to go further.
We were advised not to drive in case of complications en-route, but that was in ref to these big bleeds. If it was more of a regular period-level bleed we would just phone the birthing centre and make our way there.
It's worth stressing again that I'm in the minority and a lot of people, even with full covering, seem to make it through without any significant bleeding at all. But... if it's in the back of your mind, and you have a plan to deal with it, I think it's easier to cope if it does happen.