I had the same question when I was first starting out. If anything I could have used one or two more ribbons, but I guess we just make do with what we have. Here’s where I place them.
Proper of Seasons
You will want a ribbon here as you progress through the liturgical year.
This is the four week cycle of the psalms for Morning and Evening Prayer. I think this is the most obvious place for a ribbon as your progress through the cycle.
This is a weekly cycle, but it is handy to have a ribbon in there some where if you pray Night Prayer.
Proper of Saints
In addition to the seasons of the year, there are certain feasts that are always on the same date. This kind of creates two distinct ways by which the Church moves through the liturgical calendar. It’s good to have a ribbon in here.
Particularly in the single volume it would be nice to even have two ribbons here. The commons are those antiphons and prayers which might be common to more than one saint (i.e. common of virgins, apostles or martyrs). You will want at least one ribbon here, but sometimes a common will call for the use of another common for part of it, so if you have an extra ribbon put it in there.
I also have a few prayer cards that double as book markers in my breviary in a few other places. I will list them below as well as a few others I don’t mark, but you might find useful:
As needed, see the note above.
Sunday Week I
Someone mentioned this, but it gets used a lot.
This is the small section just after the Proper of Seasons, and before the Psalter. It is particularly helpful when you are just starting out because it is the rubrics (basically a how-to) on the proper flow of the Liturgy of the Hours.
I don’t really pray it much, but if I did, I imagine I’d want a ribbon or something there.
I find I put the ribbons in pretty much the same place in the four volume as well. Hope this helps.