Now onto the inside. The last & final water-tight seal is created between the window & flashing that wraps onto the framing. Install foam backer rod & exterior caulking; Fortiflash makes a great caulk for this application, along with a handful of others. This is
around all four sides, preventing any water from to enter as well as forming a great air infiltration barrier (no drafts!)Next is to cut & install the surround. This is the 4 pieces that extend from the inside of the window to the outer edge of the drywall. For this project we are using pre-primed Hemlock fir. We are still maintaining the integrity of the age/appearance of the home; an extended sill with an apron below as well as a back band around the edge (available at specialty millwork shops- McCoy in Portland is my preferred supplier). I cut the sill & header pieces to the width of
window- not tight to the framing. Then the sides are cut; length of window less the width of the material doubled up (1 3/8" off the total in this case). The sides and top are as wide plus a 1/16" as the amount of framing & drywall to be covered, & the bottom is however much further out your extended sill will project into the room. You also need to leave the sides as long as they are going to be in the end. Make the cuts from the bottom, so the over cut won't show. I use a 12" miter saw along with a portable table saw w/a miter guide for most of my trim jobs. I use a 2" 18 gauge brad nail to build the surround. It is then installed resting on shims 16"o/c. Spray foam is applied to the gap between the 1x6 & the framing, ensuring no drafts or air-infiltration. 
Now the legs & lentil are cut to size. the legs should extend from the sill, where they will rest, to 1/4" above the bottom of the window surround. On interior trim, I recommend a 1/4" reveal on all casings. The lentil rests on top of the legs & extends from the one outer edge to the other. Make sure to get your joints flush with each other and add a shim if necessary.
Another preference that I have is to chamfer interior butt-joints.
We refer to this as "celebrating" the joint. In reality, without a lot of work, they are not going to meet up & blend together acceptably, so I prefer to bring more attention and add a purposeful shadow line detail. If you opt to not do this, you will need to rip the eased edge of your flat stock (squaring it), add a biscuit, and prepare for some light sanding and filling. And I still find that over the years these joints have a tendency to telegraph through the paint... your call.
I now add back-band. Mitered corners are a must, and take your time to measure and cut accurately. The same rule applies here as on the exterior: a little too long is preferable. Glue the corners & pin them together on the floor, then install it like an applied picture frame. I use 23 gauge headless pins, but in the past I have used a 23 gauge wire narrow crown staple or a brad nail in a pinch. I use a 2 1/2" 15 gauge trim nail to pin the casing to the wall solidly, especially when casing doors.
Now set any nails if required. I fill the holes with light-weight spackle and a putty knife (or a chisel if I forgot to grab everything I should have).
Tape any edge you desire a nice crisp line at, apply a bead of caulk along all the linear joints. Work fast & don't let the caulk skin over before you have a chance to finger it. I like Miller's painter's caulk for interior finish situations, but most of the acrylic latex caulks would work fine. Get to know what's available and what cure's without too much shrinkage, which ones skin too quickly, etc. Peel the tape back on itself while the caulk is still wet. Let it dry for a day. Inspect & light sanding where it will help & paint.
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