I'm new to these games, but I can say that most things are the same as in CW2 and EAW, which you've already played.
The ledger, for example, you get via F1, as in those games.
To cut losses during winter I've been placing my forces inside towns (not just in the province but inside the towns) in completely static mode (green-green stance buttons) for the entire duration of winter, doing nothing. I'm unsure whether overstacking affects attrition within towns from the effects of winter, but it certainly seems to affect things when plagues and typhus and such like arrive, which they regularly do. Disease can really decimate (or worse) big stacks. Hence, I start planning to winter big armies as early as September, knowing that to avoid them being overstacked in the towns I will have to disperse them to several towns. You can read the overstacking situation (as you probably know) by selecting the stack in the bottom bar when it is inside the town, then hovering over the little circle icon that appears at the very bottom in the little info box to the bottom far right of your screen (the box which shows the orders activation envelope and command penalties etc). That will tell you what the stacking limits are in the town you're in and how many you have in there.
The supply system is the 'simple' system, meaning you don't have to try to build chains of depots to move supply through, as in CW2. Supply is taken from the province the army is in and from any controlled adjoining province (someone can correct me if I'm wrong about this), and of course from any supply wagons you have with you. You can increase supply in these provinces by building depots, which will mean that once the province becomes pillaged (which it will quite quickly if you have a big army living off it) you will still have your depot stocks to live off. If you want more supply than this you have to manually send supply wagons on trips to get it (to replenish themselves) from controlled provinces, preferably with depots). As I said, someone can correct me if I'm wrong about this.
Replacements work the same as in the other games you've already played.