Once your interior doors are installed, the next step is to fit the baseboards and casings.
I am still finishing the baseboards and casings on my house, as weather forced us to move in before we were completely finished.
You may want to add crown moldings to add some interest. Extruded foam crown molding may even provide some extra insulation along the top of exterior walls. This is a corner area that is difficult or expensive to insulate well.
See to your tools first. Make trial cuts, and fits, to make sure saws are perfectly accurate. Check you owners manual or the internet if adjustments are needed. Make certain you have appropriate blades for this type of work. They must be sharp and true.
You will likely be surprised by the amount of material needed to finish a house. The cost can be substantial.
I chose to make my own baseboards and casings from 5/8 inch MDF. I used a plain flat profile in a 3.5 inch width. I will not be installing crown moldings. This is quick and inexpensive. I needed about $200 for material as opposed to a $1000 or more for moldings.
Because MDF is heavy, and hard to handle in sheets, I used a light, cordless circular saw with a guide, for cutting the strips. It worked quite well, but a straight edge and clamps would probably have been just a little more accurate. I sanded out the saw marks, and eased the corners, using 60 grit sandpaper and an orbital sander.
On doors visible from the main living area, I added a small crown. It was made from 5/8 MDF using a router. Simple and quick. On these, I used butt joints at the top with about 1/2 inch overlap on the ends. The result, though simple, is quite attractive. Most other joints were done with a mitre.
I prefer to put on primer and a coat of paint before applying the trim. Ready made moldings from MDF or pine will be primed for you. With wood grain I apply the stain and at least one coat of varnish or urethane. Caulking or wood filler and a final coat of paint or varnish is applied after installation. For a fine finish, fill and sand nail holes and imperfect joints twice before the final coat. On wood grain use a filler that closely matches your stain in color. Filler rarely takes stain well. I use paintable caulking for painted trim. it is easy and fast.
Start with the door and window casings. Measure the inside of frames and jambs and make your cuts a little longer to leave a relieve. I like to leave about a 1/8 inch relief, but it is up to you. Up to about a 1/4 inch will leave adequate nailing space. Use 1.5 inch brads or finishing nails to attach to the frames. An occasional nail into the rough frame will hold things flat. A nail may be needed diagonally at the corners for a flush fit. A quick swipe or two with a sanding block across your end cuts may make the joints a little tighter. Use care.
Baseboard requires near perfect cuts for a neat job. If the height of your baseboards is 3.5 inches or less, you can make your cuts on a 12 inch mitre saw with the boards on edge. To make a 45 degree cut across the face of a board takes extra care, if it is laid flat on the saw. The saw can be easily forced out of true. It may be easier to use a circular saw.
Outside corners will usually fit well with 45 degree cuts. You can remove imperfections in the wall with a knife or sandpaper. Ease the edge a bit.
Make joins over studs if possible with 45 degree cuts and two nails through both boards. Use 2 inch brads or small diameter finishing nails.
A 45 degree joint rarely fits well on inside corners. With painted trim you could simply use caulking to correct the imperfections. A more professional way is to use a coped fit. This is a butt joint cut to fit the profile of your trim. To make perfect fit, cut your board at a 45, and cut with a coping saw along the cut line on the face. Emphasize the line with a pencil if necessary. It is possible to cope using the mitre saw alone. it takes care.
A flat profile means no coping cuts are neccessary.A simple butt joint can be used on the inside corners of baseboard.
Crown moldings are a little different. It may take a little practice on scrap until you can visualise the cuts. Crowns usually do not fit tightly into corners and are a little more tolerant of imperfect drywall finishing. Inside corners will be done with 45 degree joints.
It is possible that you have a few corners that are other than 45 degrees. Your mitre saw will have settings to accommodate these cases.
I like to caulk the top of baseboards, along casings, and any joints that are not perfectly tight. This eliminates hiding places for spiders, flies, and other bugs. It also provides a more professional finish. Use a paintable caulk of an appropriate color. In bathrooms, or on any floor that may be mopped, I caulk the bottom of baseboards with colored silicone to prevent water damage. It must be done carefully as silicone will not accept a finish.
A final caution or two. Wear safety glasses. Brad nailers have a habit of sending sharp projectiles flying at times. Watch you fingers. It is harder to judge where the saw blade will be when making angle cuts. A brad nailer can hit a knot, or nail, and do a 180 right into your finger. Do not attempt to pull brads if ends protrude. Cut them off with a good side cutter, preferably just below the surface.
Remember, if you cut a board too short, you will have to cut a new one. If you cut slightly long, it can easily be trimmed with a power mitre saw.