Finnish-American carpentry around windows can be done in many ways. First, you might simply drywall the sill. Second, you can build a kind of picture frame around the window. Not surprisingly this is called a picture frame casing. Or you can build in an extended sill and put on an apron...that is put an apron on the window.
I like the extended sill idea. I have never been in a house with sufficient horizontal surface area. A hundred book shelves, thirty cupboards and seventeen closets could be put into a home and it would not prevent the horizontal surfaces from quickly being overwhelmed.
There is an economic dictum that states: the human animal tends to spend to the limit of his or her income. (Many spend beyond the limit of their income.) All of this spending must be going somewhere. I figure people are spending their hard earned cash on the law of storage space, which states: the amount of junk in a home will expand until it overflows the closet and shelf area available (or the owner runs out of money). So you can never have enough shelf space - or window sills for that matter.
An extended sill will give you more space for house plants, knickknacks and holiday decorations. A sill is also relatively easy to make. Simply cut a board approximately four inches wider than the width of your window opening. Notch out each end of the board to the depth and width your window sets in the opening. Slide in the sill and nail it into the 2X4 below.
An apron is a board nailed flat against the wall beneath the sill. This will mask the jagged edge of the drywall where it meets the window. If you have chosen a very large sill, you can extend brackets from the apron to hold the sill in place. You might even put in a window seat that has storage area beneath.
Wainscoting is a fad that has been around for a couple hundred years. It involves panels installed halfway up the wall. These are topped with a chair rail.
It is very attractive in dining rooms and studies. Wainscoting protects the lower half of the wall from leaning chairs and kicking feet. However, it is proven to be completely ineffective against a child with a crayon.
American-Finnish carpentry also involves banisters, balusters, rosettes and crown molding, all of which sound like they would fit well on a medieval French castle along with flying buttresses, escarpments, and crenelated parapets. If this fancy French effect is what you are trying to achieve, just remember: the fancier you are, the longer it will take to get to the next stage of construction...and the more likely it is that someone is going to call you a pansy to your face; better just to stick with Finnish carpentry.