Welcome aboard Greg. My wife is an artist so I build a lot of frames. I struggled with it for some time before learning about the Lion Miter Trimmer, pix below. This tool was first devised somewhere in the 1880-90s. Cutting perfect 45 degree miters with both short and long parts of the frame being exactly the same length was a losing deal untill I found one made by Grizzly, https://www.amazon.com/Grizzly-G169...&qid=1539901215&sr=8-1&keywords=Miter+Trimmer
$200 on Amazon.
The cost is more for the identical tool from other brands. There's a couple of outrigger legs for it that are extra, but the mounting system for them is really chincy, and I don't use them. You cut a good 45 degree miter on a saw, with the piece about 1/8 th inch longer than the final size, then use the trimmer to shave off about 1/16th from each end. The stop on the base is EXACTLY 45 degrees. This picture is of another brand, but all brands are probably made in the same factory.
I think you can see from the picture how you hold the piece. Rather than using the arms with a stop, I just compart the lengths of the pairs of pieces to make sure they are the exact same length. You want them to be as close to exact as you can get, although there is perhaps a few thousandths margin of error. What you get is a glass smooth, perfectly cut miter. You can also use the trimmer to get an exact 90, but I don't use it much for that purpose, on faceframes, for example. Be very careful to only lift this tool by the small handle on front. If you grip it wrong, you could easily slice off a finger. I keep the shipping block for the installed blade in place at all times. And I mounted it on a sheet of ply with two heavy handles so I can move it around safely. Be careful!!!
Next comes fitting the corners together. There are many ways to do this, but I often wind up simply gluing the pieces on the ends lightly and mostly on the bottom edge to avoid squeeze out, which will not take the finish. I usually reinforce the glued up corners with a 1/8th thick spline. You need to make a simple spline jig to hold the frame in place as you cut a 1/8th inch slot across the corner of the frame. I use metal corner clamps for the most part. You pinch them closed with a small tool and they hold the frame in place. There are all kinds of braces and jigs to make sure the corners are square, but if your 45 miter is true, you may find you don't really need them. Sometimes I use an accurate square to line them up. You can also use a band clamp to hold them while glue sets. The Collins Spring Clamp tool is in the picture at the very bottom.
Next comes a very important pre finishing step, filling and sanding the corners. First, my secret weapon for filling is an Aussie product you can order on Amazon, TimberMate. It is available to match most woods, sands perfectly, doesn't shrink even in the tiniest of cracks or joints. I've tried many other fillers, but this one is the very best.
I no longer use ordinary sandpaper. I by far prefer the new 3M sanding sheets, which are flexible, almost rubbery. They last forever, and at 220 grit produce a very smooth surface. I no longer sand up to the 320 grit. The best part is that this stuff wraps around the shapes and stays in place on a set of sanding blocks:
These are Rockler Contour Sanding Grips, and they work in combination with the 3M sanding medium. They make it very easy to sand a frame, including where raised or recessed shapes come together at the miter corner. Using ordinary sandpaper, it is very easy to flatten an area before you realize it, and that will show up badly with a finish. You can see the inverted area to take care of the rounded areas, but you can also hold sanding medium in place on the other end to sand recessed areas. You match the grip size to the curve you're sanding. I also find it easy to use this for very minor twists (no such thing as perfectly flat frame material). These show up as a curve on one side of the joint being higher than the other side. I just sand these lightly until they match and look right. I wouldn't consider making a frame without these babies. About $15 per set. They also have flat/triangular shaped grips as well. I have and use both all the time. https://www.rockler.com/rockler-contour-sanding-grips-2
For finish, what I use most often is Minwax water based stain and usually a few light coats of wipe on poly, sometimes gloss, sometimes not. The harder the wood, the smoother the surface, and the more likely I'll use gloss.
Making your own frame materials is possible and not really that hard to do. But way beyond what I'm going to cover here. Finally, here's that Collins clamp tool so you can see how it works. It leaves a small dimple, but it usally gets removed when I cut the spline.