Well, I don't really have a simple way to do it, and my method does waste a quite a bit of material...but it is doable.
If you're looking for a coat a bit like this
, basically what I did was take a costume pattern for a long, ankle-length coat, mock it up, and had my brother wear it while I literally trimmed off the excess on the sides to get the look I wanted. I'd say find one that has the collar you want already, which will make things incredibly easier.
Make sure the coat pattern you need has the split up the back - I made the mistake of having to place my own, and it lead to some bad times.